Frontier User Guide

Note

Notable differences between Summit and Frontier:

Orion scratch filesystem

Frontier mounts Orion, a parallel filesystem based on Lustre and HPE ClusterStor, with a 679 PB usable namespace. Frontier will not mount Alpine and Summit will not mount Orion. Data will not be automatically transferred from Alpine to Orion, so we recommend that users move only needed data between the file systems with Globus.

See the Data and Storage section or this recording for more information.

Cray Programming Environment

Frontier utilizes the Cray Programming Environment. Many aspects including LMOD are similar to Summit’s environment. But Cray’s compiler wrappers and the Cray and AMD compilers are worth noting.

See the Compiling section for more information.

AMD GPUs

Each frontier node has 4 AMD MI250X accelerators with two Graphic Compute Dies (GCDs) in each accelerator. The system identifies each GCD as an independent device (so for simplicity we use the term GPU when we talk about a GCD) for a total of 8 GPUs per node (compared to Summit’s 6 Nvidia V100 GPUs per node). Each pair of GPUs is associated with a particular NUMA domain (see node diagram in Frontier Compute Nodes section) which might affect how your application should lay out data and computation.

See the AMD GPUs section for more information.

Programming Models

Since Frontier uses AMD GPUs, code written in Nvidia’s CUDA language will not work as is. They need to be converted to use HIP, which is AMD’s GPU programming framework, or should be converted to some other GPU framework that supports AMD GPUs as a backend e.g. OpenMP Offload, Kokkos, RAJA, OCCA, SYCL/DPC++ etc .

See the HIP section for more information and links about HIP.

Slurm batch scheduler

Frontier uses SchedMD’s Slurm Workload Manager for job scheduling instead of IBM’s LSF. Slurm provides similar functionality to LSF, albeit with different commands. Notable are the separation in batch script submission (sbatch) and interactive batch submission (salloc).

See the Slurm section for more infomation including a LSF to Slurm command comparison.

Srun job launcher

Frontier uses Slurm’s job launcher, srun, instead of Summit’s jsrun to launch parallel jobs within a batch script. Overall functionality is similar, but commands are notably different. Frontier’s compute node layout should also be considered when selecting job layout.

See the Srun section for more srun information, and see Process and Thread Mapping Examples for srun examples on Frontier.

OLCF Support

If you encounter any issues or have questions, please contact the OLCF via the following:

• Email us at help@olcf.ornl.gov
• Contact your OLCF liaison
• Sign-up to attend OLCF Office Hours

System Overview

Frontier is a HPE Cray EX supercomputer located at the Oak Ridge Leadership Computing Facility. With a theoretical peak double-precision performance of approximately 2 exaflops (2 quintillion calculations per second), it is the fastest system in the world for a wide range of traditional computational science applications. The system has 74 Olympus rack HPE cabinets, each with 128 AMD compute nodes, and a total of 9,408 AMD compute nodes.

Frontier Compute Nodes

Each Frontier compute node consists of [1x] 64-core AMD “Optimized 3rd Gen EPYC” CPU (with 2 hardware threads per physical core) with access to 512 GB of DDR4 memory. Each node also contains [4x] AMD MI250X, each with 2 Graphics Compute Dies (GCDs) for a total of 8 GCDs per node. The programmer can think of the 8 GCDs as 8 separate GPUs, each having 64 GB of high-bandwidth memory (HBM2E). The CPU is connected to each GCD via Infinity Fabric CPU-GPU, allowing a peak host-to-device (H2D) and device-to-host (D2H) bandwidth of 36+36 GB/s. The 2 GCDs on the same MI250X are connected with Infinity Fabric GPU-GPU with a peak bandwidth of 200 GB/s. The GCDs on different MI250X are connected with Infinity Fabric GPU-GPU in the arrangement shown in the Frontier Node Diagram below, where the peak bandwidth ranges from 50-100 GB/s based on the number of Infinity Fabric connections between individual GCDs.

Note

TERMINOLOGY:

The 8 GCDs contained in the 4 MI250X will show as 8 separate GPUs according to Slurm, ROCR_VISIBLE_DEVICES, and the ROCr runtime, so from this point forward in the quick-start guide, we will simply refer to the GCDs as GPUs.

Note

There are [4x] NUMA domains per node and [2x] L3 cache regions per NUMA for a total of [8x] L3 cache regions. The 8 GPUs are each associated with one of the L3 regions as follows:

NUMA 0:

• hardware threads 000-007, 064-071 | GPU 4
• hardware threads 008-015, 072-079 | GPU 5

NUMA 1:

• hardware threads 016-023, 080-087 | GPU 2
• hardware threads 024-031, 088-095 | GPU 3

NUMA 2:

• hardware threads 032-039, 096-103 | GPU 6
• hardware threads 040-047, 104-111 | GPU 7

NUMA 3:

• hardware threads 048-055, 112-119 | GPU 0
• hardware threads 056-063, 120-127 | GPU 1

By default, Frontier reserves the first core in each L3 cache region. Frontier uses low-noise mode, which constrains all system processes to core 0. Low-noise mode cannot be disabled by users. In addition, Frontier uses SLURM core specialization (-S 8 flag at job allocation time, e.g., sbatch) to reserve one core from each L3 cache region, leaving 56 allocatable cores. Set -S 0 at job allocation to override this setting.

Node Types

On Frontier, there are three major types of nodes you will encounter: Login, Launch, and Compute. While all of these are similar in terms of hardware (see: Frontier Compute Nodes), they differ considerably in their intended use.

Node Type	Description
Login	When you connect to Frontier, you’re placed on a login node. This is the place to write/edit/compile your code, manage data, submit jobs, etc. You should never launch parallel jobs from a login node nor should you run threaded jobs on a login node. Login nodes are shared resources that are in use by many users simultaneously.
Compute	Most of the nodes on Frontier are compute nodes. These are where your parallel job executes. They’re accessed via the srun command.

System Interconnect

The Frontier nodes are connected with [4x] HPE Slingshot 200 Gbps (25 GB/s) NICs providing a node-injection bandwidth of 800 Gbps (100 GB/s).

File Systems

Frontier is connected to Orion, a parallel filesystem based on Lustre and HPE ClusterStor, with a 679 PB usable namespace (/lustre/orion/). In addition to Frontier, Orion is available on the OLCF’s data transfer nodes, Andes, and some other smaller resources. It is not available from Summit. Data will not be automatically transferred from Alpine to Orion. Frontier also has access to the center-wide NFS-based filesystem (which provides user and project home areas). Each compute node has two 1.92TB Non-Volatile Memory storage devices. See Data and Storage for more information.

Frontier connects to the center’s High Performance Storage System (HPSS) - for user and project archival storage - users can log in to the Data Transfer Nodes (DTNs) to move data to/from HPSS.

Operating System

Frontier is running Cray OS 2.4 based on SUSE Linux Enterprise Server (SLES) version 15.4.

GPUs

Each Frontier compute node contains 4 AMD MI250X. The AMD MI250X has a peak performance of 53 TFLOPS in double-precision for modeling and simulation. Each MI250X contains 2 GPUs, where each GPU has a peak performance of 26.5 TFLOPS (double-precision), 110 compute units, and 64 GB of high-bandwidth memory (HBM2) which can be accessed at a peak of 1.6 TB/s. The 2 GPUs on an MI250X are connected with Infinity Fabric with a bandwidth of 200 GB/s (in each direction simultaneously).

Connecting

To connect to Frontier, ssh to frontier.olcf.ornl.gov. For example:

$ ssh <username>@frontier.olcf.ornl.gov

For more information on connecting to OLCF resources, see Connecting for the first time.

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Data and Storage

Transition from Alpine to Orion

• Frontier mounts Orion, a parallel filesystem based on Lustre and HPE ClusterStor, with a 679 PB usable namespace (/lustre/orion/). In addition to Frontier, Orion is available on the OLCF’s data transfer nodes, Andes, and some other smaller resources. It is not available from Summit. Frontier will not mount Alpine when Orion is in production.
• Data will not be automatically transferred from Alpine to Orion. Users should consider the data needed from Alpine and transfer it. Globus is the preferred method and there is access to both Orion and Alpine through the Globus OLCF DTN endpoint. See Globus. Use of HPSS to specifically stage data for the Alpine to Orion transfer is discouraged.
• On Alpine, there was no user-exposed concept of file striping, the process of dividing a file between the storage elements of the filesystem. Orion uses a feature called Progressive File Layout (PFL) that changes the striping of files as they grow. Because of this, we ask users not to manually adjust the file striping. If you feel the default striping behavior of Orion is not meeting your needs, please contact help@olcf.ornl.gov.
• As with Alpine, files older than 90 days are purged from Orion.  Please plan your data management and lifecycle at OLCF before generating the data.

For more detailed information about center-wide file systems and data archiving available on Frontier, please refer to the pages on Data Storage and Transfers. The subsections below give a quick overview of NFS, Lustre,and HPSS storage spaces as well as the on node NVMe “Burst Buffers” (SSDs).

NFS Filesystem

Area	Path	Type	Permissions	Quota	Backups	Purged	Retention	On Compute Nodes
User Home	/ccs/home/[userid]	NFS	User set	50 GB	Yes	No	90 days	Yes
Project Home	/ccs/proj/[projid]	NFS	770	50 GB	Yes	No	90 days	Yes

Note

Though the NFS filesystem’s User Home and Project Home areas are read/write from Frontier’s compute nodes, we strongly recommend that users launch and run jobs from the Lustre Orion parallel filesystem instead due to its larger storage capacity and superior performance. Please see below for Lustre Orion filesystem storage areas and paths.

Lustre Filesystem

Area	Path	Type	Permissions	Quota	Backups	Purged	Retention	On Compute Nodes
Member Work	/lustre/orion/[projid]/scratch/[userid]	Lustre HPE ClusterStor	700	50 TB	No	90 days	N/A	Yes
Project Work	/lustre/orion/[[projid]/proj-shared	Lustre HPE ClusterStor	770	50 TB	No	90 days	N/A	Yes
World Work	/lustre/orion/[[projid]/world-shared	Lustre HPE ClusterStor	775	50 TB	No	90 days	N/A	Yes

HPSS Archival Storage

Please note that the HPSS is not mounted directly onto Frontier nodes. There are two main methods for accessing and moving data to/from the HPSS. The first is to use the command line utilities hsi and htar. The second is to use the Globus data transfer service. See HPSS Data Archival System for more information on both of these methods.

Area	Path	Type	Permissions	Quota	Backups	Purged	Retention	On Compute Nodes
Member Archive	/hpss/prod/[projid]/users/$USER	HPSS	700	100 TB	No	No	90 days	No
Project Archive	/hpss/prod/[projid]/proj-shared	HPSS	770	100 TB	No	No	90 days	No
World Archive	/hpss/prod/[projid]/world-shared	HPSS	775	100 TB	No	No	90 days	No

Using Globus to Move Data to Orion

The following example is intended to help users who are making the transition from Summit to Frontier to move their data between Alpine GPFS and Orion Lustre. We strongly recommend using Globus for this transfer as it is the method that is most efficient for users and that causes the least contention on filesystems and data transfer nodes.

Note

Globus Warnings:

• Globus transfers do not preserve file permissions. Arriving files will have (rw-r–r–) permissions, meaning arriving file will have user read and write permissions and group and world read permissions. Note that the arriving files will not have any execute permissions, so you will need to use chmod to reset execute permissions before running a Globus-transferred executable.
• Globus will overwrite files at the destination with identically named source files. This is done without warning.
• Globus has restriction of 8 active transfers across all the users. Each user has a limit of 3 active transfers, so it is required to transfer a lot of data on each transfer than less data across many transfers.
• If a folder is constituted with mixed files including thousands of small files (less than 1MB each one), it would be better to tar the small files. Otherwise, if the files are larger, Globus will handle them.

Here is a recording of an example transfer from Alpine to Orion using Globus and the OLCF DTN: Using Globus to Move Data to Orion.

Below is a summary of the steps for data transfer given in the recording:

1. Login to globus.org using your globus ID and password. If you do not have a globusID, set one up here: Generate a globusID.

2. Once you are logged in, Globus will open the “File Manager” page. Click the left side “Collection” text field in the File Manager and type “OLCF DTN”.
3. When prompted, authenticate into the OLCF DTN endpoint using your OLCF username and PIN followed by your RSA passcode.
4. Click in the left side “Path” box in the File Manager and enter the path to your data on Alpine. For example, /gpfs/alpine/stf007/proj-shared/my_alpine_data. You should see a list of your files and folders under the left “Path” Box.
5. Click on all files or folders that you want to transfer in the list. This will highlight them.
6. Click on the right side “Collection” box in the File Manager and type “OLCF DTN”
7. Click in the right side “Path” box and enter the path where you want to put your data on Orion, for example, /lustre/orion/stf007/proj-shared/my_orion_data
8. Click the left “Start” button.
9. Click on “Activity“ in the left blue menu bar to monitor your transfer. Globus will send you an email when the transfer is complete.

NVMe

Each compute node on Frontier has [2x] 1.92TB Non-Volatile Memory (NVMe) storage devices (SSDs), colloquially known as a “Burst Buffer” with a peak sequential performance of 5500 MB/s (read) and 2000 MB/s (write). The purpose of the Burst Buffer system is to bring improved I/O performance to appropriate workloads. Users are not required to use the NVMes. Data can also be written directly to the parallel filesystem.

The NVMes on Frontier are local to each node.

NVMe Usage

To use the NVMe, users must request access during job allocation using the -C nvme option to sbatch, salloc, or srun. Once the devices have been granted to a job, users can access them at /mnt/bb/<userid>. Users are responsible for moving data to/from the NVMe before/after their jobs. Here is a simple example script:

#!/bin/bash
#SBATCH -A <projid>
#SBATCH -J nvme_test
#SBATCH -o %x-%j.out
#SBATCH -t 00:05:00
#SBATCH -p batch
#SBATCH -N 1
#SBATCH -C nvme

date

# Change directory to user scratch space (GPFS)
cd /gpfs/alpine/<projid>/scratch/<userid>

echo " "
echo "*****ORIGINAL FILE*****"
cat test.txt
echo "***********************"

# Move file from GPFS to SSD
mv test.txt /mnt/bb/<userid>

# Edit file from compute node
srun -n1 hostname >> /mnt/bb/<userid>/test.txt

# Move file from SSD back to GPFS
mv /mnt/bb/<userid>/test.txt .

echo " "
echo "*****UPDATED FILE******"
cat test.txt
echo "***********************"

And here is the output from the script:

$ cat nvme_test-<jobid>.out

*****ORIGINAL FILE*****
This is my file. There are many like it but this one is mine.
***********************

*****UPDATED FILE******
This is my file. There are many like it but this one is mine.
frontier0123
***********************

AMD GPUs

The AMD Instinct MI200 is built on advanced packaging technologies enabling two Graphic Compute Dies (GCDs) to be integrated into a single package in the Open Compute Project (OCP) Accelerator Module (OAM) in the MI250 and MI250X products. Each GCD is build on the AMD CDNA 2 architecture. A single Frontier node contains 4 MI250X OAMs for the total of 8 GCDs.

Note

The Slurm workload manager and the ROCr runtime treat each GCD as a separate GPU and visibility can be controlled using the ROCR_VISIBLE_DEVICES environment variable. Therefore, from this point on, the Frontier guide simply refers to a GCD as a GPU.

Each GPU contains 110 Compute Units (CUs) grouped in 4 Compute Engines (CEs). Physically, each GPU contains 112 CUs, but two are disabled. A command processor in each GPU receives API commands and transforms them into compute tasks. Compute tasks are managed by the 4 compute engines, which dispatch wavefronts to compute units. All wavefronts from a single workgroup are assigned to the same CU. In CUDA terminology, workgroups are “blocks”, wavefronts are “warps”, and work-items are “threads”. The terms are often used interchangeably.

The 110 CUs in each GPU deliver peak performance of 26.5 TFLOPS in double precision. Also, each GPU contains 64 GB of high-bandwidth memory (HBM2) accessible at a peak bandwidth of 1.6 TB/s. The 2 GPUs in an MI250X are connected with [4x] GPU-to-GPU Infinity Fabric links providing 200+200 GB/s of bandwidth. (Consult the diagram in the Frontier Compute Nodes section for information on how the accelerators are connected to each other, to the CPU, and to the network.

Note

The X+X GB/s notation describes bidirectional bandwidth, meaning X GB/s in each direction.

AMD vs NVIDIA Terminology

AMD	NVIDIA
Work-items or Threads	Threads
Workgroup	Block
Wavefront	Warp
Grid	Grid

We will be using these terms interchangeably as they refer to the same concepts in GPU programming, with the exception that we will only be using “wavefront” (which refers to a unit of 64 threads) instead of “warp” (which refers to a unit of 32 threads) as they mean different things.

Blocks (workgroups), Threads (work items), Grids, Wavefronts

When kernels are launched on a GPU, a “grid” of thread blocks are created, where the number of thread blocks in the grid and the number of threads within each block are defined by the programmer. The number of blocks in the grid (grid size) and the number of threads within each block (block size) can be specified in one, two, or three dimensions during the kernel launch. Each thread can be identified with a unique id within the kernel, indexed along the X, Y, and Z dimensions.

• Number of blocks that can be specified along each dimension in a grid: (2147483647, 2147483647, 2147483647)
• Max number of threads that can be specified along each dimension in a block: (1024, 1024, 1024)
  • However, the total of number of threads in a block has an upper limit of 1024 [i.e. (size of x dimension * size of y dimension * size of z dimension) cannot exceed 1024].

Each block (or workgroup) of threads is assigned to a single Compute Unit i.e. a single block won’t be split across multiple CUs. The threads in a block are scheduled in units of 64 threads called wavefronts (similar to warps in CUDA, but warps only have 32 threads instead of 64). When launching a kernel, up to 64KB of block level shared memory called the Local Data Store (LDS) can be statically or dynamically allocated. This shared memory between the threads in a block allows the threads to access block local data with much lower latency compared to using the HBM since the data is in the compute unit itself.

The Compute Unit

Each CU has 4 Matrix Core Units (the equivalent of NVIDIA’s Tensor core units) and 4 16-wide SIMD units. For a vector instruction that uses the SIMD units, each wavefront (which has 64 threads) is assigned to a single 16-wide SIMD unit such that the wavefront as a whole executes the instruction over 4 cycles, 16 threads per cycle. Since other wavefronts occupy the other three SIMD units at the same time, the total throughput still remains 1 instruction per cycle. Each CU maintains an instructions buffer for 10 wavefronts and also maintains 256 registers where each register is 64 4-byte wide entries.

HIP

The Heterogeneous Interface for Portability (HIP) is AMD’s dedicated GPU programming environment for designing high performance kernels on GPU hardware. HIP is a C++ runtime API and programming language that allows developers to create portable applications on different platforms, including the AMD MI250X. This means that developers can write their GPU applications and with very minimal changes be able to run their code in any environment. The API is very similar to CUDA, so if you’re already familiar with CUDA there is almost no additional work to learn HIP. See here for a series of tutorials on programming with HIP and also converting existing CUDA code to HIP with the hipify tools .

Things To Remember When Programming for AMD GPUs

• The MI250X has different denormal handling for FP16 and BF16 datatypes, which is relevant for ML training. Prefer using the BF16 over the FP16 datatype for ML models as you are more likely to encounter denormal values with FP16 (which get flushed to zero, causing failure in convergence for some ML models). See more in Using reduced precision (FP16 and BF16 datatypes).
• Memory can be automatically migrated to GPU from CPU on a page fault if XNACK operating mode is set. No need to explicitly migrate data or provide managed memory. This is useful if you’re migrating code from a programming model that relied on ‘unified’ or ‘managed’ memory. See more in Enabling GPU Page Migration. Information about how memory is accessed based on the allocator used and the XNACK mode can be found in Migration of Memory by Allocator and XNACK Mode.
• HIP has two kinds of memory allocations, coarse grained and fine grained, with tradeoffs between performance and coherence. Particularly relevant if you want to ues the hardware FP atomic instructions. See more in Floating-Point (FP) Atomic Operations and Coarse/Fine Grained Memory Allocations.
• FP32 atomicAdd operations on Local Data Store (i.e. block shared memory) can be slower than the equivalent FP64 operations. See more in Performance considerations for LDS FP atomicAdd().

See the Compiling section for information on compiling for AMD GPUs, and see the Tips and Tricks section for some detailed information to keep in mind to run more efficiently on AMD GPUs.

Programming Environment

Frontier users are provided with many pre-installed software packages and scientific libraries. To facilitate this, environment management tools are used to handle necessary changes to the shell.

Environment Modules (Lmod)

Environment modules are provided through Lmod, a Lua-based module system for dynamically altering shell environments. By managing changes to the shell’s environment variables (such as PATH, LD_LIBRARY_PATH, and PKG_CONFIG_PATH), Lmod allows you to alter the software available in your shell environment without the risk of creating package and version combinations that cannot coexist in a single environment.

General Usage

The interface to Lmod is provided by the module command:

Command	Description
module -t list	Shows a terse list of the currently loaded modules
module avail	Shows a table of the currently available modules
module help <modulename>	Shows help information about <modulename>
module show <modulename>	Shows the environment changes made by the <modulename> modulefile
module spider <string>	Searches all possible modules according to <string>
module load <modulename> [...]	Loads the given <modulename>(s) into the current environment
module use <path>	Adds <path> to the modulefile search cache and MODULESPATH
module unuse <path>	Removes <path> from the modulefile search cache and MODULESPATH
module purge	Unloads all modules
module reset	Resets loaded modules to system defaults
module update	Reloads all currently loaded modules

Searching for Modules

Modules with dependencies are only available when the underlying dependencies, such as compiler families, are loaded. Thus, module avail will only display modules that are compatible with the current state of the environment. To search the entire hierarchy across all possible dependencies, the spider sub-command can be used as summarized in the following table.

Command	Description
module spider	Shows the entire possible graph of modules
module spider <modulename>	Searches for modules named <modulename> in the graph of possible modules
module spider <modulename>/<version>	Searches for a specific version of <modulename> in the graph of possible modules
module spider <string>	Searches for modulefiles containing <string>

Compilers

Cray, AMD, and GCC compilers are provided through modules on Frontier. The Cray and AMD compilers are both based on LLVM/Clang. There is also a system/OS versions of GCC available in /usr/bin. The table below lists details about each of the module-provided compilers. Please see the following Compiling section for more detailed inforation on how to compile using these modules.

Cray Programming Environment and Compiler Wrappers

Cray provides PrgEnv-<compiler> modules (e.g., PrgEnv-cray) that load compatible components of a specific compiler toolchain. The components include the specified compiler as well as MPI, LibSci, and other libraries. Loading the PrgEnv-<compiler> modules also defines a set of compiler wrappers for that compiler toolchain that automatically add include paths and link in libraries for Cray software. Compiler wrappers are provided for C (cc), C++ (CC), and Fortran (ftn).

Note

Use the -craype-verbose flag to display the full include and link information used by the Cray compiler wrappers. This must be called on a file to see the full output (e.g., CC -craype-verbose test.cpp).

MPI

The MPI implementation available on Frontier is Cray’s MPICH, which is “GPU-aware” so GPU buffers can be passed directly to MPI calls.

---

Compiling

Compilers

Cray, AMD, and GCC compilers are provided through modules on Frontier. The Cray and AMD compilers are both based on LLVM/Clang. There is also a system/OS versions of GCC available in /usr/bin. The table below lists details about each of the module-provided compilers.

Note

It is highly recommended to use the Cray compiler wrappers (cc, CC, and ftn) whenever possible. See the next section for more details.

Vendor	Programming Environment	Compiler Module	Language	Compiler Wrapper	Compiler
Cray	PrgEnv-cray	cce	C	cc	craycc
			C++	CC	craycxx or crayCC
			Fortran	ftn	crayftn
AMD	PrgEnv-amd	amd	C	cc	amdclang
			C++	CC	amdclang++
			Fortran	ftn	amdflang
GCC	PrgEnv-gnu	gcc	C	cc	${GCC_PATH}/bin/gcc
			C++	CC	${GCC_PATH}/bin/g++
			Fortran	ftn	${GCC_PATH}/bin/gfortran

Cray Programming Environment and Compiler Wrappers

Cray provides PrgEnv-<compiler> modules (e.g., PrgEnv-cray) that load compatible components of a specific compiler toolchain. The components include the specified compiler as well as MPI, LibSci, and other libraries. Loading the PrgEnv-<compiler> modules also defines a set of compiler wrappers for that compiler toolchain that automatically add include paths and link in libraries for Cray software. Compiler wrappers are provided for C (cc), C++ (CC), and Fortran (ftn).

For example, to load the AMD programming environment, do:

module load PrgEnv-amd

This module will setup your programming environment with paths to software and libraries that are compatible with AMD compilers.

Note

Use the -craype-verbose flag to display the full include and link information used by the Cray compiler wrappers. This must be called on a file to see the full output (e.g., CC -craype-verbose test.cpp).

Exposing The ROCm Toolchain to your Programming Environment

If you need to add the tools and libraries related to ROCm, the framework for targeting AMD GPUs, to your path, you will need to use a version of ROCm that is compatible with your programming environment.

The following modules help you expose the ROCm Toolchain to your programming Environment:

Programming Environment Module	Module that gets you ROCm Toolchain	How you load it:
PrgEnv-amd	amd	amd  is loaded automatically with module load PrgEnv-amd
PrgEnv-cray or PrgEnv-gnu	amd-mixed	module load amd-mixed

Note

Both the CCE and ROCm compilers are Clang-based, so please be sure to use consistent (major) Clang versions when using them together. You can check which version of Clang is being used with CCE and ROCm by giving the --version flag to CC and hipcc, respectively.

MPI

The MPI implementation available on Frontier is Cray’s MPICH, which is “GPU-aware” so GPU buffers can be passed directly to MPI calls.

Implementation	Module	Compiler	Header Files & Linking
Cray MPICH	cray-mpich	cc, CC, ftn (Cray compiler wrappers)	MPI header files and linking is built into the Cray compiler wrappers
		hipcc	-L${MPICH_DIR}/lib -lmpi ${CRAY_XPMEM_POST_LINK_OPTS} -lxpmem ${PE_MPICH_GTL_DIR_amd_gfx90a} ${PE_MPICH_GTL_LIBS_amd_gfx90a} -I${MPICH_DIR}/include

Note

hipcc requires the ROCm Toolclain, See Exposing The ROCm Toolchain to your Programming Environment

GPU-Aware MPI

To use GPU-aware Cray MPICH, with Frontier’s PrgEnv modules, users must set the following modules and environment variables:

If using PrgEnv-amd:

module load craype-accel-amd-gfx90a

export MPICH_GPU_SUPPORT_ENABLED=1

If using PrgEnv-cray:

module load craype-accel-amd-gfx90a
module load amd-mixed

export MPICH_GPU_SUPPORT_ENABLED=1

Note

There are extra steps needed to enable GPU-aware MPI on Frontier, which depend on the compiler that is used (see 1. and 2. below).

1. Compiling with the Cray compiler wrappers, cc or CC

When using GPU-aware Cray MPICH with the Cray compiler wrappers, most of the needed libraries are automatically linked through the environment variables.

Though, the following header files and libraries must be included explicitly:

-I${ROCM_PATH}/include
-L${ROCM_PATH}/lib -lamdhip64

where the include path implies that #include <hip/hip_runtime.h> is included in the source file.

2. Compiling with hipcc

To use hipcc with GPU-aware Cray MPICH, use the following environment variables to setup the needed header files and libraries.

-I${MPICH_DIR}/include
-L${MPICH_DIR}/lib -lmpi \
  ${CRAY_XPMEM_POST_LINK_OPTS} -lxpmem \
  ${PE_MPICH_GTL_DIR_amd_gfx90a} ${PE_MPICH_GTL_LIBS_amd_gfx90a}

HIPFLAGS = --amdgpu-target=gfx90a

Determining the Compatibility of Cray MPICH and ROCm

Releases of cray-mpich are each built with a specific version of ROCm, and compatibility across multiple versions is not guaranteed. OLCF will maintain compatible default modules when possible. If using non-default modules, you can determine compatibility by reviewing the Product and OS Dependencies section in the cray-mpich release notes. This can be displayed by running module show cray-mpich/<version>. If the notes indicate compatibility with AMD ROCM X.Y or later, only use rocm/X.Y.Z modules. If using a non-default version of cray-mpich, you must add ${CRAY_MPICH_ROOTDIR}/gtl/lib to either your LD_LIBRARY_PATH at run time or your executable’s rpath at build time.

The compatibility table below was determined by linker testing with all current combinations of cray-mpich and ROCm-related modules on Frontier.

cray-mpich	ROCm
8.1.17	5.4.0, 5.3.0, 5.2.0, 5.1.0
8.1.23	5.4.0, 5.3.0, 5.2.0, 5.1.0

OpenMP

This section shows how to compile with OpenMP using the different compilers covered above.

Vendor	Module	Language	Compiler	OpenMP flag (CPU thread)
Cray	cce	C, C++	cc (wraps craycc) CC (wraps crayCC)	-fopenmp
		Fortran	ftn (wraps crayftn)	-homp -fopenmp (alias)
AMD	amd	C C++ Fortran	cc (wraps amdclang) CC (wraps amdclang++) ftn (wraps amdflang)	-fopenmp
GCC	gcc	C C++ Fortran	cc (wraps $GCC_PATH/bin/gcc) CC (wraps $GCC_PATH/bin/g++) ftn (wraps $GCC_PATH/bin/gfortran)	-fopenmp

OpenMP GPU Offload

This section shows how to compile with OpenMP Offload using the different compilers covered above.

Note

Make sure the craype-accel-amd-gfx90a module is loaded when using OpenMP offload.

Vendor	Module	Language	Compiler	OpenMP flag (GPU)
Cray	cce	C C++	cc (wraps craycc) CC (wraps crayCC)	-fopenmp
		Fortran	ftn (wraps crayftn)	-homp -fopenmp (alias)
AMD	amd	C C++ Fortran	cc (wraps amdclang) CC (wraps amdclang++) ftn (wraps amdflang) hipcc (requires flags below)	-fopenmp

Note

If invoking amdclang, amdclang++, or amdflang directly for openmp offload, or using hipcc you will need to add:

-fopenmp -target x86_64-pc-linux-gnu -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a.

HIP

This section shows how to compile HIP codes using the Cray compiler wrappers and hipcc compiler driver.

Note

Make sure the craype-accel-amd-gfx90a module is loaded when compiling HIP with the Cray compiler wrappers.

Compiler	Compile/Link Flags, Header Files, and Libraries
CC Only with PrgEnv-cray PrgEnv-amd	CFLAGS = -std=c++11 -D__HIP_ROCclr__ -D__HIP_ARCH_GFX90A__=1 --rocm-path=${ROCM_PATH} --offload-arch=gfx90a -x hip LFLAGS = --rocm-path=${ROCM_PATH} -L${ROCM_PATH}/lib -lamdhip64
hipcc	Can be used directly to compile HIP source files. To see what is being invoked within this compiler driver, issue the command, hipcc --verbose To explicitly target AMD MI250X, use --amdgpu-target=gfx90a

Note

hipcc requires the ROCm Toolclain, See Exposing The ROCm Toolchain to your Programming Environment

Note

Information about compiling code for different XNACK modes (which control page migration between GPU and CPU memory) can be found in the Compiling HIP kernels for specific XNACK modes section.

HIP + OpenMP CPU Threading

This section shows how to compile HIP + OpenMP CPU threading hybrid codes.

Note

Make sure the craype-accel-amd-gfx90a module is loaded when compiling HIP with the Cray compiler wrappers.

Vendor	Compiler	Compile/Link Flags, Header Files, and Libraries
AMD/Cray	CC	CFLAGS = -std=c++11 -D__HIP_ROCclr__ -D__HIP_ARCH_GFX90A__=1 --rocm-path=${ROCM_PATH} --offload-arch=gfx90a -x hip -fopenmp LFLAGS = --rocm-path=${ROCM_PATH} -fopenmp -L${ROCM_PATH}/lib -lamdhip64
	hipcc	Can be used to directly compile HIP source files, add -fopenmp flag to enable OpenMP threading To explicitly target AMD MI250X, use --amdgpu-target=gfx90a
GNU	CC	The GNU compilers cannot be used to compile HIP code, so all HIP kernels must be separated from CPU code. During compilation, all non-HIP files must be compiled with CC while HIP kernels must be compiled with hipcc. Then linking must be performed with the CC wrapper. NOTE: When using cmake, HIP code must currently be compiled using amdclang++ instead of hipcc.

Note

hipcc requires the ROCm Toolclain, See Exposing The ROCm Toolchain to your Programming Environment

---

Running Jobs

Computational work on Frontier is performed by jobs. Jobs typically consist of several componenets:

• A batch submission script
• A binary executable
• A set of input files for the executable
• A set of output files created by the executable

In general, the process for running a job is to:

1. Prepare executables and input files.
2. Write a batch script.
3. Submit the batch script to the batch scheduler.
4. Optionally monitor the job before and during execution.

The following sections describe in detail how to create, submit, and manage jobs for execution on Frontier. Frontier uses SchedMD’s Slurm Workload Manager as the batch scheduling system.

Login vs Compute Nodes

Recall from the System Overview that Frontier contains two node types: Login and Compute. When you connect to the system, you are placed on a login node. Login nodes are used for tasks such as code editing, compiling, etc. They are shared among all users of the system, so it is not appropriate to run tasks that are long/computationally intensive on login nodes. Users should also limit the number of simultaneous tasks on login nodes (e.g. concurrent tar commands, parallel make

Compute nodes are the appropriate place for long-running, computationally-intensive tasks. When you start a batch job, your batch script (or interactive shell for batch-interactive jobs) runs on one of your allocated compute nodes.

Warning

Compute-intensive, memory-intensive, or other disruptive processes running on login nodes may be killed without warning.

Note

Unlike Summit and Titan, there are no launch/batch nodes on Frontier. This means your batch script runs on a node allocated to you rather than a shared node. You still must use the job launcher (srun) to run parallel jobs across all of your nodes, but serial tasks need not be launched with srun.

Simplified Node Layout

To easily visualize job examples (see Process and Thread Mapping Examples further below), the compute node diagram has been simplified to the picture shown below.

In the diagram, each physical core on a Frontier compute node is composed of two logical cores that are represented by a pair of blue and grey boxes. For a given physical core, the blue box represents the logical core of the first hardware thread, where the grey box represents the logical core of the second hardware thread.

Low-noise Mode Layout

Frontier uses low-noise mode and core specialization (-S flag at job allocation, e.g., sbatch). Low-noise mode constrains all system processes to core 0. Core specialization (by default, -S 8) reserves the first core in each L3 region. This prevents the user running on the core that system processes are constrained to. This also means that there are only 56 allocatable cores by default instead of 64. Therefore, this modifies the simplified node layout to:

To override this default layout (not recommended), set -S 0 at job allocation.

Slurm

Frontier uses SchedMD’s Slurm Workload Manager for scheduling and managing jobs. Slurm maintains similar functionality to other schedulers such as IBM’s LSF, but provides unique control of Frontier’s resources through custom commands and options specific to Slurm. A few important commands can be found in the conversion table below, but please visit SchedMD’s Rosetta Stone of Workload Managers for a more complete conversion reference.

Slurm documentation for each command is available via the man utility, and on the web at https://slurm.schedmd.com/man_index.html. Additional documentation is available at https://slurm.schedmd.com/documentation.html.

Some common Slurm commands are summarized in the table below. More complete examples are given in the Monitoring and Modifying Batch Jobs section of this guide.

Command	Action/Task	LSF Equivalent
squeue	Show the current queue	bjobs
sbatch	Submit a batch script	bsub
salloc	Submit an interactive job	bsub -Is $SHELL
srun	Launch a parallel job	jsrun
sinfo	Show node/partition info	bqueues or bhosts
sacct	View accounting information for jobs/job steps	bacct
scancel	Cancel a job or job step	bkill
scontrol	View or modify job configuration.	bstop, bresume, bmod

Batch Scripts

The most common way to interact with the batch system is via batch scripts. A batch script is simply a shell script with added directives to request various resoruces from or provide certain information to the scheduling system. Aside from these directives, the batch script is simply the series of commands needed to set up and run your job.

To submit a batch script, use the command sbatch myjob.sl

Consider the following batch script:

#!/bin/bash
#SBATCH -A ABC123
#SBATCH -J RunSim123
#SBATCH -o %x-%j.out
#SBATCH -t 1:00:00
#SBATCH -p batch
#SBATCH -N 1024

cd $MEMBERWORK/abc123/Run.456
cp $PROJWORK/abc123/RunData/Input.456 ./Input.456
srun ...
cp my_output_file $PROJWORK/abc123/RunData/Output.456

In the script, Slurm directives are preceded by #SBATCH, making them appear as comments to the shell. Slurm looks for these directives through the first non-comment, non-whitespace line. Options after that will be ignored by Slurm (and the shell).

Line	Description
1	Shell interpreter line
2	OLCF project to charge
3	Job name
4	Job standard output file (%x will be replaced with the job name and %j with the Job ID)
5	Walltime requested (in HH:MM:SS format). See the table below for other formats.
6	Partition (queue) to use
7	Number of compute nodes requested
8	Blank line
9	Change into the run directory
10	Copy the input file into place
11	Run the job ( add layout details )
12	Copy the output file to an appropriate location.

Interactive Jobs

Most users will find batch jobs an easy way to use the system, as they allow you to “hand off” a job to the scheduler, allowing them to focus on other tasks while their job waits in the queue and eventually runs. Occasionally, it is necessary to run interactively, especially when developing, testing, modifying or debugging a code.

Since all compute resources are managed and scheduled by Slurm, it is not possible to simply log into the system and immediately begin running parallel codes interactively. Rather, you must request the appropriate resources from Slurm and, if necessary, wait for them to become available. This is done through an “interactive batch” job. Interactive batch jobs are submitted with the salloc command. Resources are requested via the same options that are passed via #SBATCH in a regular batch script (but without the #SBATCH prefix). For example, to request an interactive batch job with the same resources that the batch script above requests, you would use salloc -A ABC123 -J RunSim123 -t 1:00:00 -p batch -N 1024. Note there is no option for an output file…you are running interactively, so standard output and standard error will be displayed to the terminal.

Common Slurm Options

The table below summarizes options for submitted jobs. Unless otherwise noted, they can be used for either batch scripts or interactive batch jobs. For scripts, they can be added on the sbatch command line or as a #BSUB directive in the batch script. (If they’re specified in both places, the command line takes precedence.) This is only a subset of all available options. Check the Slurm Man Pages for a more complete list.

Option	Example Usage	Description
-A	#SBATCH -A ABC123	Specifies the project to which the job should be charged
-N	#SBATCH -N 1024	Request 1024 nodes for the job
-t	#SBATCH -t 4:00:00	Request a walltime of 4 hours. Walltime requests can be specified as minutes, hours:minutes, hours:minuts:seconds days-hours, days-hours:minutes, or days-hours:minutes:seconds
--threads-per-core	#SBATCH --threads-per-core=2	Number of active hardware threads per core. Can be 1 or 2 (1 is default) Must be used if using --threads-per-core=2 in your srun command.
-d	#SBATCH -d afterok:12345	Specify job dependency (in this example, this job cannot start until job 12345 exits with an exit code of 0. See the Job Dependency section for more information
-C	#SBATCH -C nvme	Request the burst buffer/NVMe on each node be made available for your job. See the Burst Buffers section for more information on using them.
-J	#SBATCH -J MyJob123	Specify the job name (this will show up in queue listings)
-o	#SBATCH -o jobout.%j	File where job STDOUT will be directed (%j will be replaced with the job ID). If no -e option is specified, job STDERR will be placed in this file, too.
-e	#SBATCH -e joberr.%j	File where job STDERR will be directed (%j will be replaced with the job ID). If no -o option is specified, job STDOUT will be placed in this file, too.
--mail-type	#SBATCH --mail-type=END	Send email for certain job actions. Can be a comma-separated list. Actions include BEGIN, END, FAIL, REQUEUE, INVALID_DEPEND, STAGE_OUT, ALL, and more.
--mail-user	#SBATCH --mail-user=user@somewhere.com	Email address to be used for notifications.
--reservation	#SBATCH --reservation=MyReservation.1	Instructs Slurm to run a job on nodes that are part of the specified reservation.
-S	#SBATCH -S 8	Instructs Slurm to reserve a specific number of cores per node (default is 8). Reserved cores cannot be used by the application.
--signal	#SBATCH --signal=USR1@300	Send the given signal to a job the specified time (in seconds) seconds before the job reaches its walltime. The signal can be by name or by number (i.e. both 10 and USR1 would send SIGUSR1). Signaling a job can be used, for example, to force a job to write a checkpoint just before Slurm kills the job (note that this option only sends the signal; the user must still make sure their job script traps the signal and handles it in the desired manner). When used with sbatch, the signal can be prefixed by “B:” (e.g. --signal=B:USR1@300) to tell Slurm to signal only the batch shell; otherwise all processes will be signaled.

Slurm Environment Variables

Slurm reads a number of environment variables, many of which can provide the same information as the job options noted above. We recommend using the job options rather than environment variables to specify job options, as it allows you to have everything self-contained within the job submission script (rather than having to remember what options you set for a given job).

Slurm also provides a number of environment variables within your running job. The following table summarizes those that may be particularly useful within your job (e.g. for naming output log files):

Variable	Description
$SLURM_SUBMIT_DIR	The directory from which the batch job was submitted. By default, a new job starts in your home directory. You can get back to the directory of job submission with cd $SLURM_SUBMIT_DIR. Note that this is not necessarily the same directory in which the batch script resides.
$SLURM_JOBID	The job’s full identifier. A common use for $SLURM_JOBID is to append the job’s ID to the standard output and error files.
$SLURM_JOB_NUM_NODES	The number of nodes requested.
$SLURM_JOB_NAME	The job name supplied by the user.
$SLURM_NODELIST	The list of nodes assigned to the job.

Job States

A job will transition through several states during its lifetime. Common ones include:

State Code	State	Description
CA	Canceled	The job was canceled (could’ve been by the user or an administrator)
CD	Completed	The job completed successfully (exit code 0)
CG	Completing	The job is in the process of completing (some processes may still be running)
PD	Pending	The job is waiting for resources to be allocated
R	Running	The job is currently running

Job Reason Codes

In addition to state codes, jobs that are pending will have a “reason code” to explain why the job is pending. Completed jobs will have a reason describing how the job ended. Some codes you might see include:

Reason	Meaning
Dependency	Job has dependencies that have not been met
JobHeldUser	Job is held at user’s request
JobHeldAdmin	Job is held at system administrator’s request
Priority	Other jobs with higher priority exist for the partition/reservation
Reservation	The job is waiting for its reservation to become available
AssocMaxJobsLimit	The job is being held because the user/project has hit the limit on running jobs
ReqNodeNotAvail	The requested a particular node, but it’s currently unavailable (it’s in use, reserved, down, draining, etc.)
JobLaunchFailure	Job failed to launch (could due to system problems, invalid program name, etc.)
NonZeroExitCode	The job exited with some code other than 0

Many other states and job reason codes exist. For a more complete description, see the squeue man page (either on the system or online).

Scheduling Policy

In a simple batch queue system, jobs run in a first-in, first-out (FIFO) order. This can lead to inefficient use of the system. If a large job is the next to run, a strict FIFO queue can cause nodes to sit idle while waiting for the large job to start. Backfilling would allow smaller, shorter jobs to use those resources that would otherwise remain idle until the large job starts. With the proper algorithm, they would do so without impacting the start time of the large job. While this does make more efficient use of the system, it encourages the submission of smaller jobs.

The DOE Leadership-Class Job Mandate

As a DOE Leadership Computing Facility, OLCF has a mandate that a large portion of Frontier’s usage come from large, leadership-class (a.k.a. capability) jobs. To ensure that OLCF complies with this directive, we strongly encourage users to run jobs on Frontier that are as large as their code will allow. To that end, OLCF implements queue policies that enable large jobs to run in a timely fashion.

Note

The OLCF implements queue policies that encourage the submission and timely execution of large, leadership-class jobs on Frontier.

The basic priority mechanism for jobs waiting in the queue is the time the job has been waiting in the queue. If your jobs require resources outside these policies such as higher priority or longer walltimes, please contact help@olcf.ornl.gov

Job Priority by Node Count

Jobs are aged according to the job’s requested node count (older age equals higher queue priority). Each job’s requested node count places it into a specific bin. Each bin has a different aging parameter, which all jobs in the bin receive.

Bin	Min Nodes	Max Nodes	Max Walltime (Hours)	Aging Boost (Days)
1	5,645	9,408	12.0	15
2	1,882	5,644	12.0	10
3	184	1,881	12.0	0
4	92	183	6.0	0
5	1	91	2.0	0

batch Queue Policy

The batch queue is the default queue for production work on Frontier. Most work on Frontier is handled through this queue. The following policies are enforced for the batch queue:

• Limit of four eligible-to-run jobs per user. (Jobs in excess of this number will be held, but will move to the eligible-to-run state at the appropriate time.)
• Users may have only 100 jobs queued in the batch queue at any time (this includes jobs in all states). Additional jobs will be rejected at submit time.

debug Quality of Service Class

The debug quality of service (QOS) class can be used to access Frontier’s compute resources for short non-production debug tasks. The QOS provides a higher priority compare to jobs of the same job size bin in production queues. Production work and job chaining using the debug QOS is prohibited. Each user is limited to one job in any state at any one point. Attempts to submit multiple jobs to this QOS will be rejected upon job submission.

To submit a job to the debug QOS, add the -q debug option to your sbatch or salloc command or #SBATCH -q debug to your job script.

Allocation Overuse Policy

Projects that overrun their allocation are still allowed to run on OLCF systems, although at a reduced priority. Like the adjustment for the number of processors requested above, this is an adjustment to the apparent submit time of the job. However, this adjustment has the effect of making jobs appear much younger than jobs submitted under projects that have not exceeded their allocation. In addition to the priority change, these jobs are also limited in the amount of wall time that can be used. For example, consider that job1 is submitted at the same time as job2. The project associated with job1 is over its allocation, while the project for job2 is not. The batch system will consider job2 to have been waiting for a longer time than job1. Additionally, projects that are at 125% of their allocated time will be limited to only 3 running jobs at a time. The adjustment to the apparent submit time depends upon the percentage that the project is over its allocation, as shown in the table below:

% of Allocation Used	Priority Reduction
< 100%	none
>=100% but <=125%	30 days
> 125%	365 days

System Reservation Policy

Projects may request to reserve a set of nodes for a period of time by contacting help@olcf.ornl.gov. If the reservation is granted, the reserved nodes will be blocked from general use for a given period of time. Only users that have been authorized to use the reservation can utilize those resources. Since no other users can access the reserved resources, it is crucial that groups given reservations take care to ensure the utilization on those resources remains high. To prevent reserved resources from remaining idle for an extended period of time, reservations are monitored for inactivity. If activity falls below 50% of the reserved resources for more than (30) minutes, the reservation will be canceled and the system will be returned to normal scheduling. A new reservation must be requested if this occurs.

The requesting project’s allocation is charged according to the time window granted, regardless of actual utilization. For example, an 8-hour, 2,000 node reservation on Frontier would be equivalent to using 16,000 Frontier node-hours of a project’s allocation.

Note

Reservations should not be confused with priority requests. If quick turnaround is needed for a few jobs or for a period of time, a priority boost should be requested. A reservation should only be requested if users need to guarantee availability of a set of nodes at a given time, such as for a live demonstration at a conference.

Job Dependencies

Oftentimes, a job will need data from some other job in the queue, but it’s nonetheless convenient to submit the second job before the first finishes. Slurm allows you to submit a job with constraints that will keep it from running until these dependencies are met. These are specified with the -d option to Slurm. Common dependency flags are summarized below. In each of these examples, only a single jobid is shown but you can specify multiple job IDs as a colon-delimited list (i.e. #SBATCH -d afterok:12345:12346:12346). For the after dependency, you can optionally specify a +time value for each jobid.

Flag	Meaning (for the dependent job)
#SBATCH -d after:jobid[+time]	The job can start after the specified jobs start or are canceled. The optional +time argument is a number of minutes. If specified, the job cannot start until that many minutes have passed since the listed jobs start/are canceled. If not specified, there is no delay.
#SBATCH -d afterany:jobid	The job can start after the specified jobs have ended (regardless of exit state)
#SBATCH -d afternotok:jobid	The job can start after the specified jobs terminate in a failed (non-zero) state
#SBATCH -d afterok:jobid	The job can start after the specified jobs complete successfully (i.e. zero exit code)
#SBATCH -d singleton	Job can begin after any previously-launched job with the same name and from the same user have completed. In other words, serialize the running jobs based on username+jobname pairs.

Monitoring and Modifying Batch Jobs

scontrol hold and scontrol release: Holding and Releasing Jobs

Sometimes you may need to place a hold on a job to keep it from starting. For example, you may have submitted it assuming some needed data was in place but later realized that data is not yet available. This can be done with the scontrol hold command. Later, when the data is ready, you can release the job (i.e. tell the system that it’s now OK to run the job) with the scontrol release command. For example:

scontrol hold 12345	Place job 12345 on hold
scontrol release 12345	Release job 12345 (i.e. tell the system it’s OK to run it)

scontrol update: Changing Job Parameters

There may also be occasions where you want to modify a job that’s waiting in the queue. For example, perhaps you requested 2,000 nodes but later realized this is a different data set and only needs 1,000 nodes. You can use the scontrol update command for this. For example:

scontrol update NumNodes=1000 JobID=12345	Change job 12345’s node request to 1000 nodes
scontrol update TimeLimit=4:00:00 JobID=12345	Change job 12345’s max walltime to 4 hours

scancel: Cancel or Signal a Job

In addition to the --signal option for the sbatch/salloc commands described above, the scancel command can be used to manually signal a job. Typically, this is used to remove a job from the queue. In this use case, you do not need to specify a signal and can simply provide the jobid (i.e. scancel 12345). If you want to send some other signal to the job, use scancel the with the -s option. The -s option allows signals to be specified either by number or by name. Thus, if you want to send SIGUSR1 to a job, you would use scancel -s 10 12345 or scancel -s USR1 12345.

squeue: View the Queue

The squeue command is used to show the batch queue. You can filter the level of detail through several command-line options. For example:

squeue -l	Show all jobs currently in the queue
squeue -l -u $USER	Show all of your jobs currently in the queue

sacct: Get Job Accounting Information

The sacct command gives detailed information about jobs currently in the queue and recently-completed jobs. You can also use it to see the various steps within a batch jobs.

sacct -a -X	Show all jobs (-a) in the queue, but summarize the whole allocation instead of showing individual steps (-X)
sacct -u $USER	Show all of your jobs, and show the individual steps (since there was no -X option)
sacct -j 12345	Show all job steps that are part of job 12345
sacct -u $USER -S 2022-07-01T13:00:00 -o "jobid%5,jobname%25,nodelist%20" -X	Show all of your jobs since 1 PM on July 1, 2022 using a particular output format

scontrol show job: Get Detailed Job Information

In addition to holding, releasing, and updating the job, the scontrol command can show detailed job information via the show job subcommand. For example, scontrol show job 12345.

Srun

The default job launcher for Frontier is srun . The srun command is used to execute an MPI binary on one or more compute nodes in parallel.

Srun Format

srun  [OPTIONS... [executable [args...]]]

Single Command (non-interactive)

$ srun -A <project_id> -t 00:05:00 -p <partition> -N 2 -n 4 --ntasks-per-node=2 ./a.out
<output printed to terminal>

The job name and output options have been removed since stdout/stderr are typically desired in the terminal window in this usage mode.

srun accepts the following common options:

-N	Number of nodes
-n	Total number of MPI tasks (default is 1)
-c, --cpus-per-task=<ncpus>	Logical cores per MPI task (default is 1) When used with --threads-per-core=1: -c is equivalent to physical cores per task By default, when -c > 1, additional cores per task are distributed within one L3 region first before filling a different L3 region.
--cpu-bind=threads	Bind tasks to CPUs. threads - (default, recommended) Automatically generate masks binding tasks to threads.
--threads-per-core=<threads>	In task layout, use the specified maximum number of hardware threads per core (default is 1; there are 2 hardware threads per physical CPU core). Must also be set in salloc or sbatch if using --threads-per-core=2 in your srun command.
-m, --distribution=<value>:<value>:<value>	Specifies the distribution of MPI ranks across compute nodes, sockets (L3 regions), and cores, respectively. The default values are block:cyclic:cyclic, see man srun for more information. Currently, the distribution setting for cores (the third “<value>” entry) has no effect on Frontier
--ntasks-per-node=<ntasks>	If used without -n: requests that a specific number of tasks be invoked on each node. If used with -n: treated as a maximum count of tasks per node.
--gpus	Specify the number of GPUs required for the job (total GPUs across all nodes).
--gpus-per-node	Specify the number of GPUs per node required for the job.
--gpu-bind=closest	Binds each task to the GPU which is on the same NUMA domain as the CPU core the MPI rank is running on.
--gpu-bind=map_gpu:<list>	Bind tasks to specific GPUs by setting GPU masks on tasks (or ranks) as specified where <list> is <gpu_id_for_task_0>,<gpu_id_for_task_1>,.... If the number of tasks (or ranks) exceeds the number of elements in this list, elements in the list will be reused as needed starting from the beginning of the list. To simplify support for large task counts, the lists may follow a map with an asterisk and repetition count. (For example map_gpu:0*4,1*4)
--ntasks-per-gpu=<ntasks>	Request that there are ntasks tasks invoked for every GPU.

Below is a comparison table between srun and jsrun.

Option	jsrun (Summit)	srun (Frontier)
Number of nodes	-nnodes	-N, --nnodes
Number of tasks	defined with resource set	-n, --ntasks
Number of tasks per node	defined with resource set	--ntasks-per-node
Number of CPUs per task	defined with resource set	-c, --cpus-per-task
Number of resource sets	-n, --nrs	N/A
Number of resource sets per host	-r, --rs_per_host	N/A
Number of tasks per resource set	-a, --tasks_per_rs	N/A
Number of CPUs per resource set	-c, --cpus_per_rs	N/A
Number of GPUs per resource set	-g, --gpus_per_rs	N/A
Bind tasks to allocated CPUs	-b, --bind	--cpu-bind
Performance binding preference	-l,--latency_priority	--hint
Specify the task to resource mapping pattern	--launch_distribution	-m, --distribution

Process and Thread Mapping Examples

This section describes how to map processes (e.g., MPI ranks) and process threads (e.g., OpenMP threads) to the CPUs, GPUs, and NICs on Frontier.

Users are highly encouraged to use the CPU- and GPU-mapping programs used in the following sections to check their understanding of the job steps (i.e., srun commands) they intend to use in their actual jobs.

• For the CPU Mapping and Multithreading sections:

  A simple MPI+OpenMP “Hello, World” program (hello_mpi_omp) will be used to clarify the mappings.

• For the GPU Mapping section:

  An MPI+OpenMP+HIP “Hello, World” program (hello_jobstep) will be used to clarify the GPU mappings.

Additionally, it may be helpful to cross reference the simplified Frontier node diagram – specifically the low-noise mode diagram.

Warning

Unless specified otherwise, the examples below assume the default low-noise core specialization setting (-S 8). This means that there are only 56 allocatable cores by default instead of 64. See the Low-noise Mode Layout section for more details. Set -S 0 at job allocation to override this setting.

CPU Mapping

This subsection covers how to map tasks to the CPU without the presence of additional threads (i.e., solely MPI tasks – no additional OpenMP threads).

The intent with both of the following examples is to launch 8 MPI ranks across the node where each rank is assigned its own logical (and, in this case, physical) core. Using the -m distribution flag, we will cover two common approaches to assign the MPI ranks – in a “round-robin” (cyclic) configuration and in a “packed” (block) configuration. Slurm’s Interactive Jobs method was used to request an allocation of 1 compute node for these examples: salloc -A <project_id> -t 30 -p <parition> -N 1

Note

There are many different ways users might choose to perform these mappings, so users are encouraged to clone the hello_mpi_omp program and test whether or not processes and threads are running where intended.

8 MPI Ranks (round-robin)

Assigning MPI ranks in a “round-robin” (cyclic) manner across L3 cache regions (sockets) is the default behavior on Frontier. This mode will assign consecutive MPI tasks to different sockets before it tries to “fill up” a socket.

Recall that the -m flag behaves like: -m <node distribution>:<socket distribution>. Hence, the key setting to achieving the round-robin nature is the -m block:cyclic flag, specifically the cyclic setting provided for the “socket distribution”. This ensures that the MPI tasks will be distributed across sockets in a cyclic (round-robin) manner.

The below srun command will achieve the intended 8 MPI “round-robin” layout:

$ export OMP_NUM_THREADS=1
$ srun -N1 -n8 -c1 --cpu-bind=threads --threads-per-core=1 -m block:cyclic ./hello_mpi_omp | sort

MPI 000 - OMP 000 - HWT 001 - Node frontier00144
MPI 001 - OMP 000 - HWT 009 - Node frontier00144
MPI 002 - OMP 000 - HWT 017 - Node frontier00144
MPI 003 - OMP 000 - HWT 025 - Node frontier00144
MPI 004 - OMP 000 - HWT 033 - Node frontier00144
MPI 005 - OMP 000 - HWT 041 - Node frontier00144
MPI 006 - OMP 000 - HWT 049 - Node frontier00144
MPI 007 - OMP 000 - HWT 057 - Node frontier00144

Breaking down the srun command, we have:

• -N1: indicates we are using 1 node
• -n8: indicates we are launching 8 MPI tasks
• -c1: indicates we are assigning 1 logical core per MPI task. In this case, because of --threads-per-core=1, this also means 1 physical core per MPI task.
• --cpu-bind=threads: binds tasks to threads
• --threads-per-core=1: use a maximum of 1 hardware thread per physical core (i.e., only use 1 logical core per physical core)
• -m block:cyclic: distribute the tasks in a block layout across nodes (default), and in a cyclic (round-robin) layout across L3 sockets
• ./hello_mpi_omp: launches the “hello_mpi_omp” executable
• | sort: sorts the output

Note

Although the above command used the default settings -c1, --cpu-bind=threads, --threads-per-core=1 and -m block:cyclic, it is always better to be explicit with your srun command to have more control over your node layout. The above command is equivalent to srun -N1 -n8.

As you can see in the node diagram above, this results in the 8 MPI tasks (outlined in different colors) being distributed “vertically” across L3 sockets.

7 MPI Ranks (packed)

Instead, you can assign MPI ranks so that the L3 regions are filled in a “packed” (block) manner. This mode will assign consecutive MPI tasks to the same L3 region (socket) until it is “filled up” or “packed” before assigning a task to a different socket.

Recall that the -m flag behaves like: -m <node distribution>:<socket distribution>. Hence, the key setting to achieving the round-robin nature is the -m block:block flag, specifically the block setting provided for the “socket distribution”. This ensures that the MPI tasks will be distributed in a packed manner.

The below srun command will achieve the intended 7 MPI “packed” layout:

$ export OMP_NUM_THREADS=1
$ srun -N1 -n7 -c1 --cpu-bind=threads --threads-per-core=1 -m block:block ./hello_mpi_omp | sort

MPI 000 - OMP 000 - HWT 001 - Node frontier00144
MPI 001 - OMP 000 - HWT 002 - Node frontier00144
MPI 002 - OMP 000 - HWT 003 - Node frontier00144
MPI 003 - OMP 000 - HWT 004 - Node frontier00144
MPI 004 - OMP 000 - HWT 005 - Node frontier00144
MPI 005 - OMP 000 - HWT 006 - Node frontier00144
MPI 006 - OMP 000 - HWT 007 - Node frontier00144

Breaking down the srun command, the only difference than the previous example is:

• -m block:block: distribute the tasks in a block layout across nodes (default), and in a block (packed) socket layout

As you can see in the node diagram above, this results in the 7 MPI tasks (outlined in different colors) being distributed “horizontally” within a socket, rather than being spread across different L3 sockets like with the previous example. However, if an 8th task was requested it would be assigned to the next L3 region on core 009.

Multithreading

Because a Frontier compute node has two hardware threads available (2 logical cores per physical core), this enables the possibility of multithreading your application (e.g., with OpenMP threads). Although the additional hardware threads can be assigned to additional MPI tasks, this is not recommended. It is highly recommended to only use 1 MPI task per physical core and to use OpenMP threads instead on any additional logical cores gained when using both hardware threads.

The following examples cover multithreading with hybrid MPI+OpenMP applications. In these examples, Slurm’s Interactive Jobs method was used to request an allocation of 1 compute node: salloc -A <project_id> -t 30 -p <parition> -N 1

Note

There are many different ways users might choose to perform these mappings, so users are encouraged to clone the hello_mpi_omp program and test whether or not processes and threads are running where intended.

2 MPI ranks - each with 2 OpenMP threads

In this example, the intent is to launch 2 MPI ranks, each of which spawn 2 OpenMP threads, and have all of the 4 OpenMP threads run on different physical CPU cores.

First (INCORRECT) attempt

To set the number of OpenMP threads spawned per MPI rank, the OMP_NUM_THREADS environment variable can be used. To set the number of MPI ranks launched, the srun flag -n can be used.

$ export OMP_NUM_THREADS=2
$ srun -N1 -n2 ./hello_mpi_omp | sort

WARNING: Requested total thread count and/or thread affinity may result in
oversubscription of available CPU resources!  Performance may be degraded.
Explicitly set OMP_WAIT_POLICY=PASSIVE or ACTIVE to suppress this message.
Set CRAY_OMP_CHECK_AFFINITY=TRUE to print detailed thread-affinity messages.
WARNING: Requested total thread count and/or thread affinity may result in
oversubscription of available CPU resources!  Performance may be degraded.
Explicitly set OMP_WAIT_POLICY=PASSIVE or ACTIVE to suppress this message.
Set CRAY_OMP_CHECK_AFFINITY=TRUE to print detailed thread-affinity messages.

MPI 000 - OMP 000 - HWT 001 - Node frontier001
MPI 000 - OMP 001 - HWT 001 - Node frontier001
MPI 001 - OMP 000 - HWT 009 - Node frontier001
MPI 001 - OMP 001 - HWT 009 - Node frontier001

The first thing to notice here is the WARNING about oversubscribing the available CPU cores. Also, the output shows each MPI rank did spawn 2 OpenMP threads, but both OpenMP threads ran on the same logical core (for a given MPI rank). This was not the intended behavior; each OpenMP thread was meant to run on its own physical CPU core.

The problem here arises from two default settings; 1) each MPI rank is only allocated 1 logical core (-c 1) and, 2) only 1 hardware thread per physical CPU core is enabled (--threads-per-core=1). When using --threads-per-core=1 and --cpu-bind=threads (the default setting), 1 logical core in -c is equivalent to 1 physical core. So in this case, each MPI rank only has 1 physical core (with 1 hardware thread) to run on - including any threads the process spawns - hence the WARNING and undesired behavior.

Second (CORRECT) attempt

Recall that in this scenario, because of the --threads-per-core=1 setting, 1 logical core is equivalent to 1 physical core when using -c. Therefore, in order for each OpenMP thread to run on its own physical CPU core, each MPI rank should be given 2 physical CPU cores (-c 2). Now the OpenMP threads will be mapped to unique hardware threads on separate physical CPU cores.

$ export OMP_NUM_THREADS=2
$ srun -N1 -n2 -c2 ./hello_mpi_omp | sort

MPI 000 - OMP 000 - HWT 001 - Node frontier001
MPI 000 - OMP 001 - HWT 002 - Node frontier001
MPI 001 - OMP 000 - HWT 009 - Node frontier001
MPI 001 - OMP 001 - HWT 010 - Node frontier001

Now the output shows that each OpenMP thread ran on its own physical CPU core. More specifically (see the Frontier Compute Node diagram), OpenMP thread 000 of MPI rank 000 ran on logical core 001 (i.e., physical CPU core 01), OpenMP thread 001 of MPI rank 000 ran on logical core 002 (i.e., physical CPU core 02), OpenMP thread 000 of MPI rank 001 ran on logical core 009 (i.e., physical CPU core 09), and OpenMP thread 001 of MPI rank 001 ran on logical core 010 (i.e., physical CPU core 10) - as intended.

Third attempt - Using multiple threads per core

To use both available hardware threads per core, the job must be allocated with --threads-per-core=2 (as opposed to only the job step - i.e., srun command). That value will then be inherited by srun unless explcitly overridden with --threads-per-core=1. Because we are using --threads-per-core=2, the usage of -c goes back to purely meaning the amount of logical cores (i.e., it is no longer equivalent to 1 physical core).

$ salloc -N1 -A <project_id> -t <time> -p <partition> --threads-per-core=2

$ export OMP_NUM_THREADS=2
$ srun -N1 -n2 -c2 ./hello_mpi_omp | sort

MPI 000 - OMP 000 - HWT 001 - Node frontier001
MPI 000 - OMP 001 - HWT 065 - Node frontier001
MPI 001 - OMP 000 - HWT 009 - Node frontier001
MPI 001 - OMP 001 - HWT 073 - Node frontier001

Comparing this output to the Frontier Compute Node diagram, we see that each pair of OpenMP threads is contained within a single physical core. MPI rank 000 ran on logical cores 001 and 065 (i.e. physical CPU core 01) and MPI rank 001 ran on logical cores 009 and 073 (i.e. physical CPU core 09).

GPU Mapping

In this sub-section, an MPI+OpenMP+HIP “Hello, World” program (hello_jobstep) will be used to show how to make only specific GPUs available to processes - which we will refer to as “GPU mapping”. Again, Slurm’s Interactive Jobs method was used to request an allocation of 2 compute nodes for these examples: salloc -A <project_id> -t 30 -p <parition> -N 2. The CPU mapping part of this example is very similar to the example used above in the Multithreading sub-section, so the focus here will be on the GPU mapping part.

In general, GPU mapping can be accomplished in different ways. For example, an application might map GPUs to MPI ranks programmatically within the code using, say, hipSetDevice. In this case, there might not be a need to map GPUs using Slurm (since it can be done in the code itself). However, many applications expect only 1 GPU to be available to each rank. It is this latter case that the following examples refer to.

Also, recall that the CPU cores in a given L3 cache region are connected to a specific GPU (see the Frontier Node Diagram and subsequent Note on NUMA domains for more information). In the examples below, knowledge of these details will be assumed.

Note

There are many different ways users might choose to perform these mappings, so users are encouraged to clone the hello_jobstep program and test whether processes and threads are mapped to the CPU cores and GPUs as intended..

Warning

Due to the unique architecture of Frontier compute nodes and the way that Slurm currently allocates GPUs and CPU cores to job steps, it is suggested that all 8 GPUs on a node are allocated to the job step to ensure that optimal bindings are possible.

hello_jobstep output

Before jumping into the examples, it is helpful to understand the output from the hello_jobstep program:

ID	Description
MPI	MPI rank ID
OMP	OpenMP thread ID
HWT	CPU hardware thread the MPI rank or OpenMP thread ran on
Node	Compute node the MPI rank or OpenMP thread ran on
GPU_ID	GPU ID the MPI rank or OpenMP thread had access to (This is the node-level, or global, GPU ID as shown in the Frontier node diagram) NOTE: This is read from ROCR_VISIBLE_DEVICES. If this variable is not set, the value of GPU_ID will be set to N/A by the program
RT_GPU_ID	The runtime GPU ID (This is the GPU ID as seen from the HIP runtime - e.g., as reported by hipGetDevice) NOTE: The HIP runtime relabels the GPUs each rank can access starting at 0
Bus_ID	The physical Bus ID associated with a GPU (The Bus ID can be used to e.g., confirm unique GPUs are being used)

Mapping 1 GPU per task

In the following examples, 1 GPU will be mapped to each MPI rank (and any OpenMP threads it might spawn). The relevant srun options for GPU mapping used in these examples are:

Slurm Option	Description
--gpus-per-task	Specify the number of GPUs required for the job on each task. This option requires an explicit task count, e.g. -n
--gpu-bind=closest	Bind each task to the GPU(s) which are closest. Here, closest refers to the GPU connected to the L3 where the MPI rank is mapped to.

Example 1: 8 MPI ranks - each with 7 CPU cores and 1 GPU (single-node)

The most common use case for running on Frontier is to run with 8 MPI ranks per node, where each rank has access to 7 physical CPU cores and 1 GPU (recall it is 7 CPU cores here instead of 8 due to core specialization: see low-noise mode diagram). The MPI rank can use the 7 CPU cores to e.g., spawn OpenMP threads on (if OpenMP CPU threading is available in the application). Here is an example of such a job step on a single node:

$ OMP_NUM_THREADS=7 srun -N1 -n8 -c7 --gpus-per-task=1 --gpu-bind=closest ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 001 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 001 - HWT 002 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 002 - HWT 003 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 003 - HWT 004 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 004 - HWT 005 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 005 - HWT 006 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 006 - HWT 007 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 001 - OMP 000 - HWT 009 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 001 - HWT 010 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 002 - HWT 011 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 003 - HWT 012 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 004 - HWT 013 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 005 - HWT 014 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 006 - HWT 015 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 002 - OMP 000 - HWT 017 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 001 - HWT 018 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 002 - HWT 019 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 003 - HWT 020 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 004 - HWT 021 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 005 - HWT 022 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 006 - HWT 023 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 003 - OMP 000 - HWT 025 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 001 - HWT 026 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 002 - HWT 027 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 003 - HWT 028 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 004 - HWT 029 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 005 - HWT 030 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 006 - HWT 031 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 004 - OMP 000 - HWT 033 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 001 - HWT 034 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 002 - HWT 035 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 003 - HWT 036 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 004 - HWT 037 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 005 - HWT 038 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 006 - HWT 039 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 005 - OMP 000 - HWT 041 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 001 - HWT 042 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 002 - HWT 043 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 003 - HWT 044 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 004 - HWT 045 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 005 - HWT 046 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 006 - HWT 047 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 006 - OMP 000 - HWT 049 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 001 - HWT 050 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 002 - HWT 051 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 003 - HWT 052 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 004 - HWT 053 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 005 - HWT 054 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 006 - HWT 055 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 007 - OMP 000 - HWT 057 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 001 - HWT 058 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 002 - HWT 059 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 003 - HWT 060 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 004 - HWT 061 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 005 - HWT 062 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 006 - HWT 063 - Node frontier00256 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

As has been pointed out previously in the Frontier documentation, notice that GPUs are NOT mapped to MPI ranks in sequential order (e.g., MPI rank 0 is mapped to physical CPU cores 1-7 and GPU 4, MPI rank 1 is mapped to physical CPU cores 9-15 and GPU 5), but this IS expected behavior. It is simply a consequence of the Frontier node architectures as shown in the Frontier Node Diagram and subsequent Note on NUMA domains.

Example 2: 1 MPI rank with 7 CPU cores and 1 GPU (single-node)

When new users first attempt to run their application on Frontier, they often want to test with 1 MPI rank that has access to 7 CPU cores and 1 GPU. Although the job step used here is very similar to Example 1, the behavior is different:

$ OMP_NUM_THREADS=7 srun -N1 -n1 -c7 --gpus-per-task=1 --gpu-bind=closest ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 049 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 000 - OMP 001 - HWT 050 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 000 - OMP 002 - HWT 051 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 000 - OMP 003 - HWT 052 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 000 - OMP 004 - HWT 053 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 000 - OMP 005 - HWT 054 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 000 - OMP 006 - HWT 055 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1

Notice that our MPI rank did not get mapped to CPU cores 1-7 and GPU 4, but instead to GPU 0 and CPU cores 49-55. The apparent reason for this can be found in the --gpu-bind section in the srun man page: GPU binding is ignored if there is only one task.. Here, Slurm appears to give the first GPU it sees and maps it to the CPU cores that are closest. So although the mapping doesn’t occur as expected, the rank is still mapped to the correct GPU given the CPU cores it ran on.

Example 3: 16 MPI ranks - each with 7 CPU cores and 1 GPU (multi-node)

This example simply extends Example 1 to run on 2 nodes, which simply requires changing the number of nodes to 2 (-N2) and the number of MPI ranks to 16 (-n16).

$ OMP_NUM_THREADS=7 srun -N2 -n16 -c7 --gpus-per-task=1 --gpu-bind=closest ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 001 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 001 - HWT 002 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 002 - HWT 003 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 003 - HWT 004 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 004 - HWT 005 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 005 - HWT 006 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 000 - OMP 006 - HWT 007 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 001 - OMP 000 - HWT 009 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 001 - HWT 010 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 002 - HWT 011 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 003 - HWT 012 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 004 - HWT 013 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 005 - HWT 014 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 001 - OMP 006 - HWT 015 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 002 - OMP 000 - HWT 017 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 001 - HWT 018 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 002 - HWT 019 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 003 - HWT 020 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 004 - HWT 021 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 005 - HWT 022 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 002 - OMP 006 - HWT 023 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 003 - OMP 000 - HWT 025 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 001 - HWT 026 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 002 - HWT 027 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 003 - HWT 028 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 004 - HWT 029 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 005 - HWT 030 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 003 - OMP 006 - HWT 031 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 004 - OMP 000 - HWT 033 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 001 - HWT 034 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 002 - HWT 035 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 003 - HWT 036 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 004 - HWT 037 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 005 - HWT 038 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 004 - OMP 006 - HWT 039 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 005 - OMP 000 - HWT 041 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 001 - HWT 042 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 002 - HWT 043 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 003 - HWT 044 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 004 - HWT 045 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 005 - HWT 046 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 005 - OMP 006 - HWT 047 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 006 - OMP 000 - HWT 049 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 001 - HWT 050 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 002 - HWT 051 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 003 - HWT 052 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 004 - HWT 053 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 005 - HWT 054 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 006 - OMP 006 - HWT 055 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 007 - OMP 000 - HWT 057 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 001 - HWT 058 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 002 - HWT 059 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 003 - HWT 060 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 004 - HWT 061 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 005 - HWT 062 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 007 - OMP 006 - HWT 063 - Node frontier04086 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 008 - OMP 000 - HWT 001 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 008 - OMP 001 - HWT 002 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 008 - OMP 002 - HWT 003 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 008 - OMP 003 - HWT 004 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 008 - OMP 004 - HWT 005 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 008 - OMP 005 - HWT 006 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 008 - OMP 006 - HWT 007 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 009 - OMP 000 - HWT 009 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 009 - OMP 001 - HWT 010 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 009 - OMP 002 - HWT 011 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 009 - OMP 003 - HWT 012 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 009 - OMP 004 - HWT 013 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 009 - OMP 005 - HWT 014 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 009 - OMP 006 - HWT 015 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 010 - OMP 000 - HWT 017 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 010 - OMP 001 - HWT 018 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 010 - OMP 002 - HWT 019 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 010 - OMP 003 - HWT 020 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 010 - OMP 004 - HWT 021 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 010 - OMP 005 - HWT 022 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 010 - OMP 006 - HWT 023 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 011 - OMP 000 - HWT 025 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 011 - OMP 001 - HWT 026 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 011 - OMP 002 - HWT 027 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 011 - OMP 003 - HWT 028 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 011 - OMP 004 - HWT 029 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 011 - OMP 005 - HWT 030 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 011 - OMP 006 - HWT 031 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 012 - OMP 000 - HWT 033 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 012 - OMP 001 - HWT 034 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 012 - OMP 002 - HWT 035 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 012 - OMP 003 - HWT 036 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 012 - OMP 004 - HWT 037 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 012 - OMP 005 - HWT 038 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 012 - OMP 006 - HWT 039 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 013 - OMP 000 - HWT 041 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 013 - OMP 001 - HWT 042 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 013 - OMP 002 - HWT 043 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 013 - OMP 003 - HWT 044 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 013 - OMP 004 - HWT 045 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 013 - OMP 005 - HWT 046 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 013 - OMP 006 - HWT 047 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 014 - OMP 000 - HWT 049 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 001 - HWT 050 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 002 - HWT 051 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 003 - HWT 052 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 004 - HWT 053 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 005 - HWT 054 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 006 - HWT 055 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 015 - OMP 000 - HWT 057 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 001 - HWT 058 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 002 - HWT 059 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 003 - HWT 060 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 004 - HWT 061 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 005 - HWT 062 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 006 - HWT 063 - Node frontier04087 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

Mapping multiple MPI ranks to a single GPU

In the following examples, 2 MPI ranks will be mapped to 1 GPU. For brevity, OMP_NUM_THREADS will be set to 1, so -c1 will be used unless otherwise specified. A new srun option will also be introduced to accomplish the new mapping:

Slurm Option	Description
--ntasks-per-gpu	Specifies the number of MPI ranks that will share access to a GPU.

Note

On AMD’s MI250X, multi-process service (MPS) is not needed since multiple MPI ranks per GPU is supported natively.

Example 4: 16 MPI ranks - where 2 ranks share a GPU (round-robin, single-node)

This example launches 16 MPI ranks (-n16), each with 1 physical CPU core (-c1) to launch 1 OpenMP thread (OMP_NUM_THREADS=1) on. The MPI ranks will be assigned to GPUs in a round-robin fashion so that each of the 8 GPUs on the node are shared by 2 MPI ranks.

$ OMP_NUM_THREADS=1 srun -N1 -n16 -c1 --ntasks-per-gpu=2 --gpu-bind=closest ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 001 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 001 - OMP 000 - HWT 009 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 002 - OMP 000 - HWT 017 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 003 - OMP 000 - HWT 025 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 004 - OMP 000 - HWT 033 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 005 - OMP 000 - HWT 041 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 006 - OMP 000 - HWT 049 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 007 - OMP 000 - HWT 057 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 008 - OMP 000 - HWT 002 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 009 - OMP 000 - HWT 010 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 010 - OMP 000 - HWT 018 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 011 - OMP 000 - HWT 026 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 012 - OMP 000 - HWT 034 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 013 - OMP 000 - HWT 042 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 014 - OMP 000 - HWT 050 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 015 - OMP 000 - HWT 058 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

The output shows the round-robin (cyclic) distribution of MPI ranks to GPUs. In fact, it is a round-robin distribution of MPI ranks to L3 cache regions (the default distribution). The GPU mapping is a consequence of where the MPI ranks are distributed; --gpu-bind=closest simply maps the GPU in an L3 cache region to the MPI ranks in the same L3 region.

Example 5: 32 MPI ranks - where 2 ranks share a GPU (round-robin, multi-node)

This example is an extension of Example 4 to run on 2 nodes.

$ OMP_NUM_THREADS=1 srun -N2 -n32 -c1 --ntasks-per-gpu=2 --gpu-bind=closest ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 001 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 001 - OMP 000 - HWT 009 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 002 - OMP 000 - HWT 017 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 003 - OMP 000 - HWT 025 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 004 - OMP 000 - HWT 033 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 005 - OMP 000 - HWT 041 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 006 - OMP 000 - HWT 049 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 007 - OMP 000 - HWT 057 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 008 - OMP 000 - HWT 002 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 009 - OMP 000 - HWT 010 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 010 - OMP 000 - HWT 018 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 011 - OMP 000 - HWT 026 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 012 - OMP 000 - HWT 034 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 013 - OMP 000 - HWT 042 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 014 - OMP 000 - HWT 050 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 015 - OMP 000 - HWT 058 - Node frontier04975 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 016 - OMP 000 - HWT 001 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 017 - OMP 000 - HWT 009 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 018 - OMP 000 - HWT 017 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 019 - OMP 000 - HWT 025 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 020 - OMP 000 - HWT 033 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 021 - OMP 000 - HWT 041 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 022 - OMP 000 - HWT 049 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 023 - OMP 000 - HWT 057 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 024 - OMP 000 - HWT 002 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 025 - OMP 000 - HWT 010 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 026 - OMP 000 - HWT 018 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 027 - OMP 000 - HWT 026 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 028 - OMP 000 - HWT 034 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 029 - OMP 000 - HWT 042 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 030 - OMP 000 - HWT 050 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 031 - OMP 000 - HWT 058 - Node frontier04976 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

Example 6: 16 MPI ranks - where 2 ranks share a GPU (packed, single-node)

Warning

This example assumes the use of a core specialization of -S 0. Because Frontier’s default core specialization (-S 8) reserves the first core in each L3 region, the “packed” mode can be problematic because the 7 cores available in each L3 region won’t necessarily divide evenly. This can lead to tasks potentially spanning multiple L3 regions with its assigned cores, which creates problems when Slurm tries to assign GPUs to a given task.

This example launches 16 MPI ranks (-n16), each with 4 physical CPU cores (-c4) to launch 1 OpenMP thread (OMP_NUM_THREADS=1) on. The MPI ranks will be assigned to GPUs in a packed fashion so that each of the 8 GPUs on the node are shared by 2 MPI ranks. Similar to Example 4, -ntasks-per-gpu=2 will be used, but a new srun flag will be used to change the default round-robin (cyclic) distribution of MPI ranks across NUMA domains:

Slurm Option	Description
--distribution=<value>[:<value>][:<value>]	Specifies the distribution of MPI ranks across compute nodes, sockets (L3 cache regions on Frontier), and cores, respectively. The default values are block:cyclic:cyclic, which is where the cyclic assignment comes from in the previous examples.

Note

In the job step for this example, --distribution=*:block is used, where * represents the default value of block for the distribution of MPI ranks across compute nodes and the distribution of MPI ranks across L3 cache regions has been changed to block from its default value of cyclic.

Note

Because the distribution across L3 cache regions has been changed to a “packed” (block) configuration, caution must be taken to ensure MPI ranks end up in the L3 cache regions where the GPUs they intend to be mapped to are located. To accomplish this, the number of physical CPU cores assigned to an MPI rank was increased - in this case to 4. Doing so ensures that only 2 MPI ranks can fit into a single L3 cache region. If the value of -c was left at 1, all 8 MPI ranks would be “packed” into the first L3 region, where the “closest” GPU would be GPU 4 - the only GPU in that L3 region.

$ OMP_NUM_THREADS=1 srun -N1 -n16 -c4 --ntasks-per-gpu=2 --gpu-bind=closest --distribution=*:block ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 000 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 001 - OMP 000 - HWT 004 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 002 - OMP 000 - HWT 008 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 003 - OMP 000 - HWT 012 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 004 - OMP 000 - HWT 016 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 005 - OMP 000 - HWT 020 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 006 - OMP 000 - HWT 024 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 007 - OMP 000 - HWT 028 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 008 - OMP 000 - HWT 032 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 009 - OMP 000 - HWT 036 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 010 - OMP 000 - HWT 040 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 011 - OMP 000 - HWT 044 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 012 - OMP 000 - HWT 048 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 013 - OMP 000 - HWT 052 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 000 - HWT 056 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 000 - HWT 060 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

The overall effect of using --distribution=*:block and increasing the number of physical CPU cores available to each MPI rank is to place the first two MPI ranks in the first L3 cache region with GPU 4, the next two MPI ranks in the second L3 cache region with GPU 5, and so on.

Example 7: 32 MPI ranks - where 2 ranks share a GPU (packed, multi-node)

Warning

This example assumes the use of a core specialization of -S 0. Because Frontier’s default core specialization (-S 8) reserves the first core in each L3 region, the “packed” mode can be problematic because the 7 cores available in each L3 region won’t necessarily divide evenly. This can lead to tasks potentially spanning multiple L3 regions with its assigned cores, which creates problems when Slurm tries to assign GPUs to a given task.

This example is an extension of Example 6 to use 2 compute nodes. With the appropriate changes put in place in Example 6, it is a straightforward exercise to change to using 2 nodes (-N2) and 32 MPI ranks (-n32).

$ OMP_NUM_THREADS=1 srun -N2 -n32 -c4 --ntasks-per-gpu=2 --gpu-bind=closest --distribution=*:block ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 000 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 001 - OMP 000 - HWT 004 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 002 - OMP 000 - HWT 010 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 003 - OMP 000 - HWT 012 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 004 - OMP 000 - HWT 016 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 005 - OMP 000 - HWT 021 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 006 - OMP 000 - HWT 024 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 007 - OMP 000 - HWT 028 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 008 - OMP 000 - HWT 032 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 009 - OMP 000 - HWT 037 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 010 - OMP 000 - HWT 041 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 011 - OMP 000 - HWT 044 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 012 - OMP 000 - HWT 049 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 013 - OMP 000 - HWT 052 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 014 - OMP 000 - HWT 056 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 015 - OMP 000 - HWT 060 - Node frontier002 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 016 - OMP 000 - HWT 000 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 017 - OMP 000 - HWT 004 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 018 - OMP 000 - HWT 008 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 019 - OMP 000 - HWT 013 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 020 - OMP 000 - HWT 016 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 021 - OMP 000 - HWT 020 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 022 - OMP 000 - HWT 024 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 023 - OMP 000 - HWT 028 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 024 - OMP 000 - HWT 034 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 025 - OMP 000 - HWT 036 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 026 - OMP 000 - HWT 040 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 027 - OMP 000 - HWT 044 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 028 - OMP 000 - HWT 048 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 029 - OMP 000 - HWT 052 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 030 - OMP 000 - HWT 056 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 031 - OMP 000 - HWT 060 - Node frontier004 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

Example 8: 56 MPI ranks - where 7 ranks share a GPU (packed, single-node)

An alternative solution to Example 6 and 7’s -S 8 issue is to use -c 1 instead. There is no problem when running with 1 core per MPI rank (i.e., 7 ranks per GPU) because the task can’t span multiple L3s.

$ OMP_NUM_THREADS=1 srun -N1 -n56 -c1 --ntasks-per-gpu=7 --gpu-bind=closest --distribution=*:block ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 001 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 001 - OMP 000 - HWT 002 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 002 - OMP 000 - HWT 003 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 003 - OMP 000 - HWT 004 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 004 - OMP 000 - HWT 005 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 005 - OMP 000 - HWT 006 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 006 - OMP 000 - HWT 007 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
MPI 007 - OMP 000 - HWT 009 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 008 - OMP 000 - HWT 010 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 009 - OMP 000 - HWT 011 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 010 - OMP 000 - HWT 012 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 011 - OMP 000 - HWT 013 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 012 - OMP 000 - HWT 014 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 013 - OMP 000 - HWT 015 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
MPI 014 - OMP 000 - HWT 017 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 015 - OMP 000 - HWT 018 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 016 - OMP 000 - HWT 019 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 017 - OMP 000 - HWT 020 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 018 - OMP 000 - HWT 021 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 019 - OMP 000 - HWT 022 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 020 - OMP 000 - HWT 023 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
MPI 021 - OMP 000 - HWT 025 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 022 - OMP 000 - HWT 026 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 023 - OMP 000 - HWT 027 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 024 - OMP 000 - HWT 028 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 025 - OMP 000 - HWT 029 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 026 - OMP 000 - HWT 030 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 027 - OMP 000 - HWT 031 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
MPI 028 - OMP 000 - HWT 033 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 029 - OMP 000 - HWT 034 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 030 - OMP 000 - HWT 035 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 031 - OMP 000 - HWT 036 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 032 - OMP 000 - HWT 037 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 033 - OMP 000 - HWT 038 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 034 - OMP 000 - HWT 039 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
MPI 035 - OMP 000 - HWT 041 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 036 - OMP 000 - HWT 042 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 037 - OMP 000 - HWT 043 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 038 - OMP 000 - HWT 044 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 039 - OMP 000 - HWT 045 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 040 - OMP 000 - HWT 046 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 041 - OMP 000 - HWT 047 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de
MPI 042 - OMP 000 - HWT 049 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 043 - OMP 000 - HWT 050 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 044 - OMP 000 - HWT 051 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 045 - OMP 000 - HWT 052 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 046 - OMP 000 - HWT 053 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 047 - OMP 000 - HWT 054 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 048 - OMP 000 - HWT 055 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
MPI 049 - OMP 000 - HWT 057 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 050 - OMP 000 - HWT 058 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 051 - OMP 000 - HWT 059 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 052 - OMP 000 - HWT 060 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 053 - OMP 000 - HWT 061 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 054 - OMP 000 - HWT 062 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
MPI 055 - OMP 000 - HWT 063 - Node frontier08413 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

Multiple Independent Job Steps

Example 9: 8 independent and simultaneous job steps running on a single node

This example shows how to run multiple independent, simultaneous job steps on a single compute node. Specifically, it shows how to run 8 independent hello_jobstep programs running on their own CPU core and GPU.

Submission script:

#!/bin/bash

#SBATCH -A stf016_frontier
#SBATCH -N 1
#SBATCH -t 5

for idx in {1..8};

    do
        date

        OMP_NUM_THREADS=1 srun -u --gpus-per-task=1 --gpu-bind=closest -N1 -n1 -c1 ./hello_jobstep &

        sleep 1
    done

wait

Output:

Fri 02 Jun 2023 03:33:45 PM EDT
MPI 000 - OMP 000 - HWT 049 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1
Fri 02 Jun 2023 03:33:46 PM EDT
MPI 000 - OMP 000 - HWT 057 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6
Fri 02 Jun 2023 03:33:47 PM EDT
MPI 000 - OMP 000 - HWT 017 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 2 - Bus_ID c9
Fri 02 Jun 2023 03:33:48 PM EDT
MPI 000 - OMP 000 - HWT 025 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 3 - Bus_ID ce
Fri 02 Jun 2023 03:33:49 PM EDT
MPI 000 - OMP 000 - HWT 001 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 4 - Bus_ID d1
Fri 02 Jun 2023 03:33:50 PM EDT
MPI 000 - OMP 000 - HWT 009 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 5 - Bus_ID d6
Fri 02 Jun 2023 03:33:51 PM EDT
MPI 000 - OMP 000 - HWT 033 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 6 - Bus_ID d9
Fri 02 Jun 2023 03:33:52 PM EDT
MPI 000 - OMP 000 - HWT 041 - Node frontier04724 - RT_GPU_ID 0 - GPU_ID 7 - Bus_ID de

The output shows that each independent process ran on its own CPU core and GPU on the same single node. To show that the ranks ran simultaneously, date was called before each job step and a 20 second sleep was added to the end of the hello_jobstep program. So the output also shows that the first job step was submitted at :45 and the subsequent job steps were all submitted between :46 and :52. But because each hello_jobstep sleeps for 20 seconds, the subsequent job steps must have all been running while the first job step was still sleeping (and holding up its resources). And the same argument can be made for the other job steps.

Note

The wait command is needed so the job script (and allocation) do not immediately end after launching the job steps in the background.

The sleep 1 is needed to give Slurm sufficient time to launch each job step.

Multiple GPUs per MPI rank

As mentioned previously, all GPUs are accessible by all MPI ranks by default, so it is possible to programatically map any combination of GPUs to MPI ranks. It should be noted however that Cray MPICH does not support GPU-aware MPI for multiple GPUs per rank, so this binding is not suggested.

NIC Mapping

As shown in the Frontier Node Diagram, each of the 4 NICs on a compute node is connected to a specific MI250X, and each MI250X is (in turn) connected to a specific NUMA domain - so each NUMA domain is correlated to a specific NIC. By default, processes (e.g., MPI ranks) that are mapped to CPU cores in a specific NUMA domain are mapped (by CrayMPICH) to the NIC that is correlated to that NUMA domain.

Note

If a user attempts to map a process to a set of cores that span more than 1 NUMA domain using the default NIC mapping, they will see an error such as MPICH ERROR: Unable to use a NIC_POLICY of 'NUMA'. Rank 0 is not confined to a single NUMA node.. This is expected behavior for the default NIC policy.

The default behavior can be changed by using the MPICH_OFI_NIC_POLICY environment variable (see man mpi for available options).

Tips for Launching at Scale

SBCAST your executable and libraries

Slurm contains a utility called sbcast. This program takes a file and broadcasts it to each node’s node-local storage (ie, /tmp, NVMe). This is useful for sharing large input files, binaries and shared libraries, while reducing the overhead on shared file systems and overhead at startup. This is highly recommended at scale if you have multiple shared libraries on Lustre/NFS file systems.

SBCASTing a single file

Here is a simple example of a file sbcast from a user’s scratch space on Lustre to each node’s NVMe drive:

#!/bin/bash
#SBATCH -A <projid>
#SBATCH -J sbcast_to_nvme
#SBATCH -o %x-%j.out
#SBATCH -t 00:05:00
#SBATCH -p batch
#SBATCH -N 2
#SBATCH -C nvme

date

# Change directory to user scratch space (Orion)
cd /lustre/orion/<projid>/scratch/<userid>

echo "This is an example file" > test.txt
echo
echo "*****ORIGINAL FILE*****"
cat test.txt
echo "***********************"

# SBCAST file from Orion to NVMe -- NOTE: ``-C nvme`` is required to use the NVMe drive
sbcast -pf test.txt /mnt/bb/$USER/test.txt
if [ ! "$?" == "0" ]; then
    # CHECK EXIT CODE. When SBCAST fails, it may leave partial files on the compute nodes, and if you continue to launch srun,
    # your application may pick up partially complete shared library files, which would give you confusing errors.
    echo "SBCAST failed!"
    exit 1
fi

echo
echo "*****DISPLAYING FILES ON EACH NODE IN THE ALLOCATION*****"
# Check to see if file exists
srun -N ${SLURM_NNODES} -n ${SLURM_NNODES} --ntasks-per-node=1 bash -c "echo \"\$(hostname): \$(ls -lh /mnt/bb/$USER/test.txt)\""
echo "*********************************************************"

echo
# Showing the file on the current node -- this will be the same on all other nodes in the allocation
echo "*****SBCAST FILE ON CURRENT NODE******"
cat /mnt/bb/$USER/test.txt
echo "**************************************"

and here is the output from that script:

Fri 03 Mar 2023 03:43:30 PM EST

*****ORIGINAL FILE*****
This is an example file
***********************

*****DISPLAYING FILES ON EACH NODE IN THE ALLOCATION*****
frontier00001: -rw-r--r-- 1 hagertnl hagertnl 24 Mar  3 15:43 /mnt/bb/hagertnl/test.txt
frontier00002: -rw-r--r-- 1 hagertnl hagertnl 24 Mar  3 15:43 /mnt/bb/hagertnl/test.txt
*********************************************************

*****SBCAST FILE ON CURRENT NODE******
This is an example file
**************************************

SBCASTing a binary with libraries stored on shared file systems

sbcast also handles binaries and their libraries:

#!/bin/bash
#SBATCH -A <projid>
#SBATCH -J sbcast_binary_to_nvme
#SBATCH -o %x-%j.out
#SBATCH -t 00:05:00
#SBATCH -p batch
#SBATCH -N 2
#SBATCH -C nvme

date

# Change directory to user scratch space (Orion)
cd /lustre/orion/<projid>/scratch/<userid>

# For this example, I use a HIP-enabled LAMMPS binary, with dependencies to MPI, HIP, and HWLOC
exe="lmp"

echo "*****ldd ./${exe}*****"
ldd ./${exe}
echo "*************************"

# SBCAST executable from Orion to NVMe -- NOTE: ``-C nvme`` is needed in SBATCH headers to use the NVMe drive
# NOTE: dlopen'd files will NOT be picked up by sbcast
# SBCAST automatically excludes several directories: /lib,/usr/lib,/lib64,/usr/lib64,/opt
#   - These directories are node-local and are very fast to read from, so SBCASTing them isn't critical
#   - see ``$ scontrol show config | grep BcastExclude`` for current list
#   - OLCF-provided libraries in ``/sw`` are not on the exclusion list. ``/sw`` is an NFS shared file system
#   - To override, add ``--exclude=NONE`` to arguments
sbcast --send-libs -pf ${exe} /mnt/bb/$USER/${exe}
if [ ! "$?" == "0" ]; then
    # CHECK EXIT CODE. When SBCAST fails, it may leave partial files on the compute nodes, and if you continue to launch srun,
    # your application may pick up partially complete shared library files, which would give you confusing errors.
    echo "SBCAST failed!"
    exit 1
fi

# Check to see if file exists
echo "*****ls -lh /mnt/bb/$USER*****"
ls -lh /mnt/bb/$USER/
echo "*****ls -lh /mnt/bb/$USER/${exe}_libs*****"
ls -lh /mnt/bb/$USER/${exe}_libs

# SBCAST sends all libraries detected by `ld` (minus any excluded), and stores them in the same directory in each node's node-local storage
# Any libraries opened by `dlopen` are NOT sent, since they are not known by the linker at run-time.

# At minimum: prepend the node-local path to LD_LIBRARY_PATH to pick up the SBCAST libraries
# It is also recommended that you **remove** any paths that you don't need, like those that contain the libraries that you just SBCAST'd
# Failure to remove may result in unnecessary calls to stat shared file systems
export LD_LIBRARY_PATH="/mnt/bb/$USER/${exe}_libs:${LD_LIBRARY_PATH}"

# If you SBCAST **all** your libraries (ie, `--exclude=NONE`), you may use the following line:
#export LD_LIBRARY_PATH="/mnt/bb/$USER/${exe}_libs:$(pkg-config --variable=libdir libfabric)"
# Use with caution -- certain libraries may use ``dlopen`` at runtime, and that is NOT covered by sbcast
# If you use this option, we recommend you contact OLCF Help Desk for the latest list of additional steps required

# You may notice that some libraries are still linked from /sw/frontier, even after SBCASTing.
# This is because the Spack-build modules use RPATH to find their dependencies.
echo "*****ldd /mnt/bb/$USER/${exe}*****"
ldd /mnt/bb/$USER/${exe}
echo "*************************************"

and here is the output from that script:

Tue 28 Mar 2023 05:01:41 PM EDT
*****ldd ./lmp*****
    linux-vdso.so.1 (0x00007fffeda02000)
    libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fffed5bb000)
    libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fffed398000)
    libm.so.6 => /lib64/libm.so.6 (0x00007fffed04d000)
    librt.so.1 => /lib64/librt.so.1 (0x00007fffece44000)
    libamdhip64.so.4 => /opt/rocm-4.5.2/lib/libamdhip64.so.4 (0x00007fffec052000)
    libmpi_cray.so.12 => /opt/cray/pe/lib64/libmpi_cray.so.12 (0x00007fffe96cc000)
    libmpi_gtl_hsa.so.0 => /opt/cray/pe/lib64/libmpi_gtl_hsa.so.0 (0x00007fffe9469000)
    libhsa-runtime64.so.1 => /opt/rocm-5.3.0/lib/libhsa-runtime64.so.1 (0x00007fffe8fdc000)
    libhwloc.so.15 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/hwloc-2.5.0-4p6jkgf5ez6wr27pytkzyptppzpugu3e/lib/libhwloc.so.15 (0x00007fffe8d82000)
    libdl.so.2 => /lib64/libdl.so.2 (0x00007fffe8b7e000)
    libhipfft.so => /opt/rocm-5.3.0/lib/libhipfft.so (0x00007fffed9d2000)
    libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007fffe875b000)
    libc.so.6 => /lib64/libc.so.6 (0x00007fffe8366000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fffed7da000)
    libamd_comgr.so.2 => /opt/rocm-5.3.0/lib/libamd_comgr.so.2 (0x00007fffe06e0000)
    libnuma.so.1 => /usr/lib64/libnuma.so.1 (0x00007fffe04d4000)
    libfabric.so.1 => /opt/cray/libfabric/1.15.2.0/lib64/libfabric.so.1 (0x00007fffe01e2000)
    libatomic.so.1 => /usr/lib64/libatomic.so.1 (0x00007fffdffd9000)
    libpmi.so.0 => /opt/cray/pe/lib64/libpmi.so.0 (0x00007fffdfdd7000)
    libpmi2.so.0 => /opt/cray/pe/lib64/libpmi2.so.0 (0x00007fffdfb9e000)
    libquadmath.so.0 => /usr/lib64/libquadmath.so.0 (0x00007fffdf959000)
    libmodules.so.1 => /opt/cray/pe/lib64/cce/libmodules.so.1 (0x00007fffed9b5000)
    libfi.so.1 => /opt/cray/pe/lib64/cce/libfi.so.1 (0x00007fffdf3b4000)
    libcraymath.so.1 => /opt/cray/pe/lib64/cce/libcraymath.so.1 (0x00007fffed8ce000)
    libf.so.1 => /opt/cray/pe/lib64/cce/libf.so.1 (0x00007fffed83b000)
    libu.so.1 => /opt/cray/pe/lib64/cce/libu.so.1 (0x00007fffdf2ab000)
    libcsup.so.1 => /opt/cray/pe/lib64/cce/libcsup.so.1 (0x00007fffed832000)
    libamdhip64.so.5 => /opt/rocm-5.3.0/lib/libamdhip64.so.5 (0x00007fffdda8a000)
    libelf.so.1 => /usr/lib64/libelf.so.1 (0x00007fffdd871000)
    libdrm.so.2 => /usr/lib64/libdrm.so.2 (0x00007fffdd65d000)
    libdrm_amdgpu.so.1 => /usr/lib64/libdrm_amdgpu.so.1 (0x00007fffdd453000)
    libpciaccess.so.0 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/libpciaccess-0.16-6evcvl74myxfxdrddmnsvbc7sgfx6s5j/lib/libpciaccess.so.0 (0x00007fffdd24a000)
    libxml2.so.2 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/libxml2-2.9.12-h7fe2yauotu3xit7rsaixaowd3noknur/lib/libxml2.so.2 (0x00007fffdcee6000)
    librocfft.so.0 => /opt/rocm-5.3.0/lib/librocfft.so.0 (0x00007fffdca1a000)
    libz.so.1 => /lib64/libz.so.1 (0x00007fffdc803000)
    libtinfo.so.6 => /lib64/libtinfo.so.6 (0x00007fffdc5d5000)
    libcxi.so.1 => /usr/lib64/libcxi.so.1 (0x00007fffdc3b0000)
    libcurl.so.4 => /usr/lib64/libcurl.so.4 (0x00007fffdc311000)
    libjson-c.so.3 => /usr/lib64/libjson-c.so.3 (0x00007fffdc101000)
    libpals.so.0 => /opt/cray/pe/lib64/libpals.so.0 (0x00007fffdbefc000)
    libgfortran.so.5 => /opt/cray/pe/gcc-libs/libgfortran.so.5 (0x00007fffdba50000)
    liblzma.so.5 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/xz-5.2.5-zwra4gpckelt5umczowf3jtmeiz3yd7u/lib/liblzma.so.5 (0x00007fffdb82a000)
    libiconv.so.2 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/libiconv-1.16-coz5dzhtoeq5unhjirayfn2xftnxk43l/lib/libiconv.so.2 (0x00007fffdb52e000)
    librocfft-device-0.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-0.so.0 (0x00007fffa11a0000)
    librocfft-device-1.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-1.so.0 (0x00007fff6491b000)
    librocfft-device-2.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-2.so.0 (0x00007fff2a828000)
    librocfft-device-3.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-3.so.0 (0x00007ffef7eee000)
    libnghttp2.so.14 => /usr/lib64/libnghttp2.so.14 (0x00007ffef7cc6000)
    libidn2.so.0 => /usr/lib64/libidn2.so.0 (0x00007ffef7aa9000)
    libssh.so.4 => /usr/lib64/libssh.so.4 (0x00007ffef783b000)
    libpsl.so.5 => /usr/lib64/libpsl.so.5 (0x00007ffef7629000)
    libssl.so.1.1 => /usr/lib64/libssl.so.1.1 (0x00007ffef758a000)
    libcrypto.so.1.1 => /usr/lib64/libcrypto.so.1.1 (0x00007ffef724a000)
    libgssapi_krb5.so.2 => /usr/lib64/libgssapi_krb5.so.2 (0x00007ffef6ff8000)
    libldap_r-2.4.so.2 => /usr/lib64/libldap_r-2.4.so.2 (0x00007ffef6da4000)
    liblber-2.4.so.2 => /usr/lib64/liblber-2.4.so.2 (0x00007ffef6b95000)
    libzstd.so.1 => /usr/lib64/libzstd.so.1 (0x00007ffef6865000)
    libbrotlidec.so.1 => /usr/lib64/libbrotlidec.so.1 (0x00007ffef6659000)
    libunistring.so.2 => /usr/lib64/libunistring.so.2 (0x00007ffef62d6000)
    libjitterentropy.so.3 => /usr/lib64/libjitterentropy.so.3 (0x00007ffef60cf000)
    libkrb5.so.3 => /usr/lib64/libkrb5.so.3 (0x00007ffef5df6000)
    libk5crypto.so.3 => /usr/lib64/libk5crypto.so.3 (0x00007ffef5bde000)
    libcom_err.so.2 => /lib64/libcom_err.so.2 (0x00007ffef59da000)
    libkrb5support.so.0 => /usr/lib64/libkrb5support.so.0 (0x00007ffef57cb000)
    libresolv.so.2 => /lib64/libresolv.so.2 (0x00007ffef55b3000)
    libsasl2.so.3 => /usr/lib64/libsasl2.so.3 (0x00007ffef5396000)
    libbrotlicommon.so.1 => /usr/lib64/libbrotlicommon.so.1 (0x00007ffef5175000)
    libkeyutils.so.1 => /usr/lib64/libkeyutils.so.1 (0x00007ffef4f70000)
    libselinux.so.1 => /lib64/libselinux.so.1 (0x00007ffef4d47000)
    libpcre.so.1 => /usr/lib64/libpcre.so.1 (0x00007ffef4abe000)
*************************
*****ls -lh /mnt/bb/hagertnl*****
total 236M
-rwxr-xr-x 1 hagertnl hagertnl 236M Mar 28 17:01 lmp
drwx------ 2 hagertnl hagertnl  114 Mar 28 17:01 lmp_libs
*****ls -lh /mnt/bb/hagertnl/lmp_libs*****
total 9.2M
-rwxr-xr-x 1 hagertnl hagertnl 1.6M Oct  6  2021 libhwloc.so.15
-rwxr-xr-x 1 hagertnl hagertnl 1.6M Oct  6  2021 libiconv.so.2
-rwxr-xr-x 1 hagertnl hagertnl 783K Oct  6  2021 liblzma.so.5
-rwxr-xr-x 1 hagertnl hagertnl 149K Oct  6  2021 libpciaccess.so.0
-rwxr-xr-x 1 hagertnl hagertnl 5.2M Oct  6  2021 libxml2.so.2
*****ldd /mnt/bb/hagertnl/lmp*****
    linux-vdso.so.1 (0x00007fffeda02000)
    libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fffed5bb000)
    libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fffed398000)
    libm.so.6 => /lib64/libm.so.6 (0x00007fffed04d000)
    librt.so.1 => /lib64/librt.so.1 (0x00007fffece44000)
    libamdhip64.so.4 => /opt/rocm-4.5.2/lib/libamdhip64.so.4 (0x00007fffec052000)
    libmpi_cray.so.12 => /opt/cray/pe/lib64/libmpi_cray.so.12 (0x00007fffe96cc000)
    libmpi_gtl_hsa.so.0 => /opt/cray/pe/lib64/libmpi_gtl_hsa.so.0 (0x00007fffe9469000)
    libhsa-runtime64.so.1 => /opt/rocm-5.3.0/lib/libhsa-runtime64.so.1 (0x00007fffe8fdc000)
    libhwloc.so.15 => /mnt/bb/hagertnl/lmp_libs/libhwloc.so.15 (0x00007fffe8d82000)
    libdl.so.2 => /lib64/libdl.so.2 (0x00007fffe8b7e000)
    libhipfft.so => /opt/rocm-5.3.0/lib/libhipfft.so (0x00007fffed9d2000)
    libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007fffe875b000)
    libc.so.6 => /lib64/libc.so.6 (0x00007fffe8366000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fffed7da000)
    libamd_comgr.so.2 => /opt/rocm-5.3.0/lib/libamd_comgr.so.2 (0x00007fffe06e0000)
    libnuma.so.1 => /usr/lib64/libnuma.so.1 (0x00007fffe04d4000)
    libfabric.so.1 => /opt/cray/libfabric/1.15.2.0/lib64/libfabric.so.1 (0x00007fffe01e2000)
    libatomic.so.1 => /usr/lib64/libatomic.so.1 (0x00007fffdffd9000)
    libpmi.so.0 => /opt/cray/pe/lib64/libpmi.so.0 (0x00007fffdfdd7000)
    libpmi2.so.0 => /opt/cray/pe/lib64/libpmi2.so.0 (0x00007fffdfb9e000)
    libquadmath.so.0 => /usr/lib64/libquadmath.so.0 (0x00007fffdf959000)
    libmodules.so.1 => /opt/cray/pe/lib64/cce/libmodules.so.1 (0x00007fffed9b5000)
    libfi.so.1 => /opt/cray/pe/lib64/cce/libfi.so.1 (0x00007fffdf3b4000)
    libcraymath.so.1 => /opt/cray/pe/lib64/cce/libcraymath.so.1 (0x00007fffed8ce000)
    libf.so.1 => /opt/cray/pe/lib64/cce/libf.so.1 (0x00007fffed83b000)
    libu.so.1 => /opt/cray/pe/lib64/cce/libu.so.1 (0x00007fffdf2ab000)
    libcsup.so.1 => /opt/cray/pe/lib64/cce/libcsup.so.1 (0x00007fffed832000)
    libamdhip64.so.5 => /opt/rocm-5.3.0/lib/libamdhip64.so.5 (0x00007fffdda8a000)
    libelf.so.1 => /usr/lib64/libelf.so.1 (0x00007fffdd871000)
    libdrm.so.2 => /usr/lib64/libdrm.so.2 (0x00007fffdd65d000)
    libdrm_amdgpu.so.1 => /usr/lib64/libdrm_amdgpu.so.1 (0x00007fffdd453000)
    libpciaccess.so.0 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/libpciaccess-0.16-6evcvl74myxfxdrddmnsvbc7sgfx6s5j/lib/libpciaccess.so.0 (0x00007fffdd24a000)
    libxml2.so.2 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/libxml2-2.9.12-h7fe2yauotu3xit7rsaixaowd3noknur/lib/libxml2.so.2 (0x00007fffdcee6000)
    librocfft.so.0 => /opt/rocm-5.3.0/lib/librocfft.so.0 (0x00007fffdca1a000)
    libz.so.1 => /lib64/libz.so.1 (0x00007fffdc803000)
    libtinfo.so.6 => /lib64/libtinfo.so.6 (0x00007fffdc5d5000)
    libcxi.so.1 => /usr/lib64/libcxi.so.1 (0x00007fffdc3b0000)
    libcurl.so.4 => /usr/lib64/libcurl.so.4 (0x00007fffdc311000)
    libjson-c.so.3 => /usr/lib64/libjson-c.so.3 (0x00007fffdc101000)
    libpals.so.0 => /opt/cray/pe/lib64/libpals.so.0 (0x00007fffdbefc000)
    libgfortran.so.5 => /opt/cray/pe/gcc-libs/libgfortran.so.5 (0x00007fffdba50000)
    liblzma.so.5 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/xz-5.2.5-zwra4gpckelt5umczowf3jtmeiz3yd7u/lib/liblzma.so.5 (0x00007fffdb82a000)
    libiconv.so.2 => /sw/frontier/spack-envs/base/opt/linux-sles15-x86_64/gcc-7.5.0/libiconv-1.16-coz5dzhtoeq5unhjirayfn2xftnxk43l/lib/libiconv.so.2 (0x00007fffdb52e000)
    librocfft-device-0.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-0.so.0 (0x00007fffa11a0000)
    librocfft-device-1.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-1.so.0 (0x00007fff6491b000)
    librocfft-device-2.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-2.so.0 (0x00007fff2a828000)
    librocfft-device-3.so.0 => /opt/rocm-5.3.0/lib/librocfft-device-3.so.0 (0x00007ffef7eee000)
    libnghttp2.so.14 => /usr/lib64/libnghttp2.so.14 (0x00007ffef7cc6000)
    libidn2.so.0 => /usr/lib64/libidn2.so.0 (0x00007ffef7aa9000)
    libssh.so.4 => /usr/lib64/libssh.so.4 (0x00007ffef783b000)
    libpsl.so.5 => /usr/lib64/libpsl.so.5 (0x00007ffef7629000)
    libssl.so.1.1 => /usr/lib64/libssl.so.1.1 (0x00007ffef758a000)
    libcrypto.so.1.1 => /usr/lib64/libcrypto.so.1.1 (0x00007ffef724a000)
    libgssapi_krb5.so.2 => /usr/lib64/libgssapi_krb5.so.2 (0x00007ffef6ff8000)
    libldap_r-2.4.so.2 => /usr/lib64/libldap_r-2.4.so.2 (0x00007ffef6da4000)
    liblber-2.4.so.2 => /usr/lib64/liblber-2.4.so.2 (0x00007ffef6b95000)
    libzstd.so.1 => /usr/lib64/libzstd.so.1 (0x00007ffef6865000)
    libbrotlidec.so.1 => /usr/lib64/libbrotlidec.so.1 (0x00007ffef6659000)
    libunistring.so.2 => /usr/lib64/libunistring.so.2 (0x00007ffef62d6000)
    libjitterentropy.so.3 => /usr/lib64/libjitterentropy.so.3 (0x00007ffef60cf000)
    libkrb5.so.3 => /usr/lib64/libkrb5.so.3 (0x00007ffef5df6000)
    libk5crypto.so.3 => /usr/lib64/libk5crypto.so.3 (0x00007ffef5bde000)
    libcom_err.so.2 => /lib64/libcom_err.so.2 (0x00007ffef59da000)
    libkrb5support.so.0 => /usr/lib64/libkrb5support.so.0 (0x00007ffef57cb000)
    libresolv.so.2 => /lib64/libresolv.so.2 (0x00007ffef55b3000)
    libsasl2.so.3 => /usr/lib64/libsasl2.so.3 (0x00007ffef5396000)
    libbrotlicommon.so.1 => /usr/lib64/libbrotlicommon.so.1 (0x00007ffef5175000)
    libkeyutils.so.1 => /usr/lib64/libkeyutils.so.1 (0x00007ffef4f70000)
    libselinux.so.1 => /lib64/libselinux.so.1 (0x00007ffef4d47000)
    libpcre.so.1 => /usr/lib64/libpcre.so.1 (0x00007ffef4abe000)
*************************************

Notice that the libraries are sent to the ${exe}_libs directory in the same prefix as the executable. Once libraries are here, you cannot tell where they came from, so consider doing an ldd of your executable prior to sbcast.

Alternative: SBCASTing a binary with all libraries

As mentioned above, you can alternatively use --exclude=NONE on sbcast to send all libraries along with the binary. Using --exclude=NONE requires more effort but substantially simplifies the linker configuration at run-time. A job script for the previous example, modified for sending all libraries is shown below.

#!/bin/bash
#SBATCH -A <projid>
#SBATCH -J sbcast_binary_to_nvme
#SBATCH -o %x-%j.out
#SBATCH -t 00:05:00
#SBATCH -p batch
#SBATCH -N 2
#SBATCH -C nvme

date

# Change directory to user scratch space (Orion)
cd /lustre/orion/<projid>/scratch/<userid>

# For this example, I use a HIP-enabled LAMMPS binary, with dependencies to MPI, HIP, and HWLOC
exe="lmp"

echo "*****ldd ./${exe}*****"
ldd ./${exe}
echo "*************************"

# SBCAST executable from Orion to NVMe -- NOTE: ``-C nvme`` is needed in SBATCH headers to use the NVMe drive
# NOTE: dlopen'd files will NOT be picked up by sbcast
sbcast --send-libs --exclude=NONE -pf ${exe} /mnt/bb/$USER/${exe}
if [ ! "$?" == "0" ]; then
    # CHECK EXIT CODE. When SBCAST fails, it may leave partial files on the compute nodes, and if you continue to launch srun,
    # your application may pick up partially complete shared library files, which would give you confusing errors.
    echo "SBCAST failed!"
    exit 1
fi

# Check to see if file exists
echo "*****ls -lh /mnt/bb/$USER*****"
ls -lh /mnt/bb/$USER/
echo "*****ls -lh /mnt/bb/$USER/${exe}_libs*****"
ls -lh /mnt/bb/$USER/${exe}_libs

# SBCAST sends all libraries detected by `ld` (minus any excluded), and stores them in the same directory in each node's node-local storage
# Any libraries opened by `dlopen` are NOT sent, since they are not known by the linker at run-time.

# All required libraries now reside in /mnt/bb/$USER/${exe}_libs
export LD_LIBRARY_PATH="/mnt/bb/$USER/${exe}_libs"

# libfabric dlopen's several libraries:
export LD_LIBRARY_PATH="${LD_LIBRARY_PATH}:$(pkg-config --variable=libdir libfabric)"

# cray-mpich dlopen's libhsa-runtime64.so and libamdhip64.so (non-versioned), so symlink on each node:
srun -N ${SLURM_NNODES} -n ${SLURM_NNODES} --ntasks-per-node=1 --label -D /mnt/bb/$USER/${exe}_libs \
    bash -c "if [ -f libhsa-runtime64.so.1 ]; then ln -s libhsa-runtime64.so.1 libhsa-runtime64.so; fi;
    if [ -f libamdhip64.so.5 ]; then ln -s libamdhip64.so.5 libamdhip64.so; fi"

# RocBLAS has over 1,000 device libraries that may be `dlopen`'d by RocBLAS during a run.
# It's impractical to SBCAST all of these, so you can set this path instead, if you use RocBLAS:
#export ROCBLAS_TENSILE_LIBPATH=${ROCM_PATH}/lib/rocblas/library

# You may notice that some libraries are still linked from /sw/crusher, even after SBCASTing.
# This is because the Spack-build modules use RPATH to find their dependencies. This behavior cannot be changed.
echo "*****ldd /mnt/bb/$USER/${exe}*****"
ldd /mnt/bb/$USER/${exe}
echo "*************************************"

Some libraries still resolved to paths outside of /mnt/bb, and the reason for that is that the executable may have several paths in RPATH.

---

Software

Visualization and analysis tasks should be done on the Andes cluster. There are a few tools provided for various visualization tasks, as described in the Visualization tools section of the Andes User Guide.

For a full list of software availability and latest news at the OLCF, please reference the Software section in OLCF’s User Documentation.

Debugging

Linaro DDT

Linaro DDT is an advanced debugging tool used for scalar, multi-threaded, and large-scale parallel applications. In addition to traditional debugging features (setting breakpoints, stepping through code, examining variables), DDT also supports attaching to already-running processes and memory debugging. In-depth details of DDT can be found in the Official DDT User Guide, and instructions for how to use it on OLCF systems can be found on the Debugging Software page. DDT is the OLCF’s recommended debugging software for large parallel applications.

One of the most useful features of DDT is its remote debugging feature. This allows you to connect to a debugging session on Frontier from a client running on your workstation. The local client provides much faster interaction than you would have if using the graphical client on Frontier. For guidance in setting up the remote client see the Debugging Software page.

GDB

GDB, the GNU Project Debugger, is a command-line debugger useful for traditional debugging and investigating code crashes. GDB lets you debug programs written in Ada, C, C++, Objective-C, Pascal (and many other languages).

GDB is availableon Summit under all compiler families:

module load gdb

To use GDB to debug your application run:

gdb ./path_to_executable

Additional information about GDB usage can befound on the GDB Documentation Page.

Valgrind4hpc

Valgrind4hpc is a Valgrind-based debugging tool to aid in the detection of memory leaks and errors in parallel applications. Valgrind4hpc aggregates any duplicate messages across ranks to help provide an understandable picture of program behavior. Valgrind4hpc manages starting and redirecting output from many copies of Valgrind, as well as deduplicating and filtering Valgrind messages. If your program can be debugged with Valgrind, it can be debugged with Valgrind4hpc.

Valgrind4hpc is available on Frontier under all compiler families:

module load valgrind4hpc

Additional information about Valgrind4hpc usage can be found on the HPE Cray Programming Environment User Guide Page.

Profiling Applications

Getting Started with the HPE Performance Analysis Tools (PAT)

The Performance Analysis Tools (PAT), formerly CrayPAT, are a suite of utilities that enable users to capture and analyze performance data generated during program execution. These tools provide an integrated infrastructure for measurement, analysis, and visualization of computation, communication, I/O, and memory utilization to help users optimize programs for faster execution and more efficient computing resource usage.

There are three programming interfaces available: (1) Perftools-lite, (2) Perftools, and (3) Perftools-preload.

Below are two examples that generate an instrumented executable using Perftools, which is an advanced interface that provides full-featured data collection and analysis capability, including full traces with timeline displays.

The first example generates an instrumented executable using a PrgEnv-amd build:

module load PrgEnv-amd
module load craype-accel-amd-gfx90a
module load rocm
module load perftools

export PATH="${PATH}:${ROCM_PATH}/llvm/bin"
export CXX='CC -x hip'
export CXXFLAGS='-ggdb -O3 -std=c++17 –Wall'
export LD='CC'
export LDFLAGS="${CXXFLAGS} -L${ROCM_PATH}/lib"
export LIBS='-lamdhip64'

make clean
make

pat_build -g hip,io,mpi -w -f <executable>

The second example generates an instrumened executable using a hipcc build:

module load perftools
module load craype-accel-amd-gfx90a
module load rocm

export CXX='hipcc'
export CXXFLAGS="$(pat_opts include hipcc) \
  $(pat_opts pre_compile hipcc) -g -O3 -std=c++17 -Wall \
  --offload-arch=gfx90a -I${CRAY_MPICH_DIR}/include \
  $(pat_opts post_compile hipcc)"
export LD='hipcc'
export LDFLAGS="$(pat_opts pre_link hipcc) ${CXXFLAGS} \
  -L${CRAY_MPICH_DIR}/lib ${PE_MPICH_GTL_DIR_amd_gfx908}"
export LIBS="-lmpi ${PE_MPICH_GTL_LIBS_amd_gfx908} \
  $(pat_opts post_link hipcc)"

make clean
make

pat_build -g hip,io,mpi -w -f <executable>

The pat_build command in the above examples generates an instrumented executable with +pat appended to the executable name (e.g., hello_jobstep+pat).

When run, the instrumented executable will trace HIP, I/O, MPI, and all user functions and generate a folder of results (e.g., hello_jobstep+pat+39545-2t).

To analyze these results, use the pat_report command, e.g.:

pat_report hello_jobstep+pat+39545-2t

The resulting report includes profiles of functions, profiles of maximum function times, details on load imbalance, details on program energy and power usages, details on memory high water mark, and more.

More detailed information on the HPE Performance Analysis Tools can be found in the HPE Performance Analysis Tools User Guide.

Note

When using perftools-lite-gpu, there is a known issue causing ld.lld not to be found. A workaround this issue can be found here.

Getting Started with HPCToolkit

HPCToolkit is an integrated suite of tools for measurement and analysis of program performance on computers ranging from multicore desktop systems to the nation’s largest supercomputers. HPCToolkit provides accurate measurements of a program’s work, resource consumption, and inefficiency, correlates these metrics with the program’s source code, works with multilingual, fully optimized binaries, has very low measurement overhead, and scales to large parallel systems. HPCToolkit’s measurements provide support for analyzing a program execution cost, inefficiency, and scaling characteristics both within and across nodes of a parallel system.

Programming models supported by HPCToolkit include MPI, OpenMP, OpenACC, CUDA, OpenCL, DPC++, HIP, RAJA, Kokkos, and others.

Below is an example that generates a profile and loads the results in their GUI-based viewer.

Note

A full list of available HPCToolkit versions can be seen with the module spider hpctoolkit command.

module load hpctoolkit/2022.05.15-rocm

# 1. Profile and trace an application using CPU time and GPU performance counters
srun <srun_options> hpcrun -o <measurement_dir> -t -e CPUTIME -e gpu=amd <application>

# 2. Analyze the binary of executables and its dependent libraries
hpcstruct <measurement_dir>

# 3. Combine measurements with program structure information and generate a database
hpcprof -o <database_dir> <measurement_dir>

# 4. Understand performance issues by analyzing profiles and traces with the GUI
hpcviewer <database_dir>

More detailed information on HPCToolkit can be found in the HPCToolkit User’s Manual.

Note

HPCToolkit does not require a recompile to profile the code. It is recommended to use the -g optimization flag for attribution to source lines.

Getting Started with the ROCm Profiler

rocprof gathers metrics on kernels run on AMD GPU architectures. The profiler works for HIP kernels, as well as offloaded kernels from OpenMP target offloading, OpenCL, and abstraction layers such as Kokkos. For a simple view of kernels being run, rocprof --stats --timestamp on is a great place to start. With the --stats option enabled, rocprof will generate a file that is named results.stats.csv by default, but named <output>.stats.csv if the -o flag is supplied. This file will list all kernels being run, the number of times they are run, the total duration and the average duration (in nanoseconds) of the kernel, and the GPU usage percentage. More detailed infromation on rocprof profiling modes can be found at ROCm Profiler documentation.

Note

If you are using sbcast, you need to explicitly sbcast the AQL profiling library found in ${ROCM_PATH}/hsa-amd-aqlprofile/lib/libhsa-amd-aqlprofile64.so. A symbolic link to this library can also be found in ${ROCM_PATH}/lib. Alternatively, you may leave ${ROCM_PATH}/lib in your LD_LIBRARY_PATH.

Roofline Profiling with the ROCm Profiler

The Roofline performance model is an increasingly popular way to demonstrate and understand application performance. This section documents how to construct a simple roofline model for a single kernel using rocprof. This roofline model is designed to be comparable to rooflines constructed by NVIDIA’s NSight Compute. A roofline model plots the achieved performance (in floating-point operations per second, FLOPS/s) as a function of arithmetic (or operational) intensity (in FLOPS per Byte). The model detailed here calculates the bytes moved as they move to and from the GPU’s HBM.

Note

Integer instructions and cache levels are currently not documented here.

To get started, you will need to make an input file for rocprof, to be passed in through rocprof -i <input_file> --timestamp on -o my_output.csv <my_exe>. Below is an example, and contains the information needed to roofline profile GPU 0, as seen by each rank:

pmc : TCC_EA_RDREQ_32B_sum TCC_EA_RDREQ_sum TCC_EA_WRREQ_sum TCC_EA_WRREQ_64B_sum SQ_INSTS_VALU_ADD_F16 SQ_INSTS_VALU_MUL_F16 SQ_INSTS_VALU_FMA_F16 SQ_INSTS_VALU_TRANS_F16 SQ_INSTS_VALU_ADD_F32 SQ_INSTS_VALU_MUL_F32 SQ_INSTS_VALU_FMA_F32 SQ_INSTS_VALU_TRANS_F32
pmc : SQ_INSTS_VALU_ADD_F64 SQ_INSTS_VALU_MUL_F64 SQ_INSTS_VALU_FMA_F64 SQ_INSTS_VALU_TRANS_F64 SQ_INSTS_VALU_MFMA_MOPS_F16 SQ_INSTS_VALU_MFMA_MOPS_BF16 SQ_INSTS_VALU_MFMA_MOPS_F32 SQ_INSTS_VALU_MFMA_MOPS_F64
gpu: 0

Note

In an application with more than one kernel, you should strongly consider filtering by kernel name by adding a line like: kernel: <kernel_name> to the rocprof input file.

This provides the minimum set of metrics used to construct a roofline model, in the minimum number of passes. Each line that begins with pmc indicates that the application will be re-run, and the metrics in that line will be collected. rocprof can collect up to 8 counters from each block (SQ, TCC) in each application re-run. To gather metrics across multiple MPI ranks, you will need to use a command that redirects the output of rocprof to a unique file for each task. For example:

srun -N 2 -n 16 --ntasks-per-node=8 --gpus-per-node=8 --gpu-bind=closest bash -c 'rocprof -o ${SLURM_JOBID}_${SLURM_PROCID}.csv -i <input_file> --timestamp on <exe>'

Note

The gpu: filter in the rocprof input file identifies GPUs by the number the MPI rank would see them as. In the srun example above, each MPI rank only has 1 GPU, so each rank sees its GPU as GPU 0.

Theoretical Roofline

The theoretical (not attainable) peak roofline constructs a theoretical maximum performance for each operational intensity.

Note

theoretical peak is determined by the hardware specifications and is not attainable in practice. attaiable peak is the performance as measured by in-situ microbenchmarks designed to best utilize the hardware. achieved performance is what the profiled application actually achieves.

The theoretical roofline can be constructed as:

\[FLOPS_{peak} = minimum(ArithmeticIntensity * BW_{HBM}, TheoreticalFLOPS)\]

On Frontier, the memory bandwidth for HBM is 1.6 TB/s, and the theoretical peak floating-point FLOPS/s when using vector registers is calculated by:

\[TheoreticalFLOPS = 128 FLOP/cycle/CU * 110 CU * 1700000000 cycles/second = 23.9 TFLOP/s\]

However, when using MFMA instructions, the theoretical peak floating-point FLOPS/s is calculated by:

\[TheoreticalFLOPS = flop\_per\_cycle(precision) FLOP/cycle/CU * 110 CU * 1700000000 cycles/second\]

where flop_per_cycle(precision) is the published floating-point operations per clock cycle, per compute unit. Those values are:

Data Type	Flops/Clock/CU
FP64	256
FP32	256
FP16	1024
BF16	1024
INT8	1024

Note

Attainable peak rooflines are constructed using microbenchmarks, and are not currently discussed here. Attainable rooflines consider the limitations of cooling and power consumption and are more representative of what an application can achieve.

Achieved FLOPS/s

We calculate the achieved performance at the desired level (here, double-precision floating point, FP64), by summing each metric count and weighting the FMA metric by 2, since a fused multiply-add is considered 2 floating point operations. Also note that these SQ_INSTS_VALU_<ADD,MUL,TRANS> metrics are reported as per-simd, so we mutliply by the wavefront size as well. The SQ_INSTS_VALU_MFMA_MOPS_* instructions should be multiplied by the Flops/Cycle/CU value listed above. We use this equation to calculate the number of double-precision FLOPS:

\[\begin{split}FP64\_FLOPS = 64 *&(SQ\_INSTS\_VALU\_ADD\_F64 \\\\ &+ SQ\_INSTS\_VALU\_MUL\_F64 \\\\ &+ SQ\_INSTS\_VALU\_TRANS\_F64 \\\\ &+ 2 * SQ\_INSTS\_VALU\_FMA\_F64) \\\\ + 256 *&(SQ\_INSTS\_VALU\_MFMA\_MOPS\_F64)\end{split}\]

When SQ_INSTS_VALU_MFMA_MOPS_*_F64 instructions are used, then 47.8 TF/s is considered the theoretical maximum FLOPS/s. If only SQ_INSTS_VALU_<ADD,MUL,TRANS> are found, then 23.9 TF/s is the theoretical maximum FLOPS/s. Then, we divide the number of FLOPS by the elapsed time of the kernel to find FLOPS per second. This is found from subtracting the rocprof metrics EndNs by BeginNs, provided by --timestamp on, then converting from nanoseconds to seconds by dividing by 1,000,000,000 (power(10,9)).

Note

For ROCm/5.2.0 and earlier, there is a known issue with the timings provided by --timestamp on. See Known Issues.

Calculating for all precisions

The above formula can be adapted to compute the total FLOPS across all floating-point precisions (INT excluded).

\[\begin{split}TOTAL\_FLOPS = 64 *&(SQ\_INSTS\_VALU\_ADD\_F16 \\\\ &+ SQ\_INSTS\_VALU\_MUL\_F16 \\\\ &+ SQ\_INSTS\_VALU\_TRANS\_F16 \\\\ &+ 2 * SQ\_INSTS\_VALU\_FMA\_F16) \\\\ + 64 *&(SQ\_INSTS\_VALU\_ADD\_F32 \\\\ &+ SQ\_INSTS\_VALU\_MUL\_F32 \\\\ &+ SQ\_INSTS\_VALU\_TRANS\_F32 \\\\ &+ 2 * SQ\_INSTS\_VALU\_FMA\_F32) \\\\ + 64 *&(SQ\_INSTS\_VALU\_ADD\_F64 \\\\ &+ SQ\_INSTS\_VALU\_MUL\_F64 \\\\ &+ SQ\_INSTS\_VALU\_TRANS\_F64 \\\\ &+ 2 * SQ\_INSTS\_VALU\_FMA\_F64) \\\\ + 1024 &*(SQ\_INSTS\_VALU\_MFMA\_MOPS\_F16) \\\\ + 1024 &*(SQ\_INSTS\_VALU\_MFMA\_MOPS\_BF16) \\\\ + 256 *&(SQ\_INSTS\_VALU\_MFMA\_MOPS\_F32) \\\\ + 256 *&(SQ\_INSTS\_VALU\_MFMA\_MOPS\_F64) \\\\\end{split}\]

Arithmetic Intensity

Arithmetic intensity calculates the ratio of FLOPS to bytes moved between HBM and L2 cache. We calculated FLOPS above (FP64_FLOPS). We can calculate the number of bytes moved using the rocprof metrics TCC_EA_WRREQ_64B, TCC_EA_WRREQ_sum, TCC_EA_RDREQ_32B, and TCC_EA_RDREQ_sum. TCC refers to the L2 cache, and EA is the interface between L2 and HBM. WRREQ and RDREQ are write-requests and read-requests, respectively. Each of these requests is either 32 bytes or 64 bytes. So we calculate the number of bytes traveling over the EA interface as:

\[BytesMoved = BytesWritten + BytesRead\]

where

\[BytesWritten = 64 * TCC\_EA\_WRREQ\_64B\_sum + 32 * (TCC\_EA\_WRREQ\_sum - TCC\_EA\_WRREQ\_64B\_sum)\]

\[BytesRead = 32 * TCC\_EA\_RDREQ\_32B\_sum + 64 * (TCC\_EA\_RDREQ\_sum - TCC\_EA\_RDREQ\_32B\_sum)\]

---

Tips and Tricks

This section details ‘tips and tricks’ and information of interest to users when porting from Summit to Frontier.

Using reduced precision (FP16 and BF16 datatypes)

Users leveraging BF16 and FP16 datatypes for applications such as ML/AI training and low-precision matrix multiplication should be aware that the AMD MI250X GPU has different denormal handling than the V100 GPUs on Summit. On the MI250X, the V_DOT2 and the matrix instructions for FP16 and BF16 flush input and output denormal values to zero. FP32 and FP64 MFMA instructions do not flush input and output denormal values to zero.

When training deep learning models using FP16 precision, some models may fail to converge with FP16 denorms flushed to zero. This occurs in operations encountering denormal values, and so is more likely to occur in FP16 because of a small dynamic range. BF16 numbers have a larger dynamic range than FP16 numbers and are less likely to encounter denormal values.

AMD has provided a solution in ROCm 5.0 which modifies the behavior of Tensorflow, PyTorch, and rocBLAS. This modification starts with FP16 input values, casting the intermediate FP16 values to BF16, and then casting back to FP16 output after the accumulate FP32 operations. In this way, the input and output types are unchanged. The behavior is enabled by default in machine learning frameworks. This behavior requires user action in rocBLAS, via a special enum type. For more information, see the rocBLAS link below.

If you encounter significant differences when running using reduced precision, explore replacing non-converging models in FP16 with BF16, because of the greater dynamic range in BF16. We recommend using BF16 for ML models in general. If you have further questions or encounter issues, contact help@olcf.ornl.gov.

Additional information on MI250X reduced precision can be found at:

• The MI250X ISA specification details the flush to zero denorm behavior at: https://developer.amd.com/wp-content/resources/CDNA2_Shader_ISA_18November2021.pdf (See page 41 and 46)
• AMD rocBLAS library reference guide details this behavior at: https://rocblas.readthedocs.io/en/master/API_Reference_Guide.html#mi200-gfx90a-considerations

Enabling GPU Page Migration

The AMD MI250X and operating system on Frontier supports unified virtual addressing across the entire host and device memory, and automatic page migration between CPU and GPU memory. Migratable, universally addressable memory is sometimes called ‘managed’ or ‘unified’ memory, but neither of these terms fully describes how memory may behave on Frontier. In the following section we’ll discuss how the heterogenous memory space on a Frontier node is surfaced within your application.

The accessibility of memory from GPU kernels and whether pages may migrate depends three factors: how the memory was allocated; the XNACK operating mode of the GPU; whether the kernel was compiled to support page migration. The latter two factors are intrinsically linked, as the MI250X GPU operating mode restricts the types of kernels which may run.

XNACK (pronounced X-knack) refers to the AMD GPU’s ability to retry memory accesses that fail due to a page fault. The XNACK mode of an MI250X can be changed by setting the environment variable HSA_XNACK before starting a process that uses the GPU. Valid values are 0 (disabled) and 1 (enabled), and all processes connected to a GPU must use the same XNACK setting. The default MI250X on Frontier is HSA_XNACK=0.

If HSA_XNACK=0, page faults in GPU kernels are not handled and will terminate the kernel. Therefore all memory locations accessed by the GPU must either be resident in the GPU HBM or mapped by the HIP runtime. Memory regions may be migrated between the host DDR4 and GPU HBM using explicit HIP library functions such as hipMemAdvise and hipPrefetchAsync, but memory will not be automatically migrated based on access patterns alone.

If HSA_XNACK=1, page faults in GPU kernels will trigger a page table lookup. If the memory location can be made accessible to the GPU, either by being migrated to GPU HBM or being mapped for remote access, the appropriate action will occur and the access will be replayed. Page migration will happen between CPU DDR4 and GPU HBM according to page touch. The exceptions are if the programmer uses a HIP library call such as hipPrefetchAsync to request migration, or if a preferred location is set via hipMemAdvise, or if GPU HBM becomes full and the page must forcibly be evicted back to CPU DDR4 to make room for other data.

Migration of Memory by Allocator and XNACK Mode

Most applications that use “managed” or “unified” memory on other platforms will want to enable XNACK to take advantage of automatic page migration on Frontier. The following table shows how common allocators currently behave with XNACK enabled. The behavior of a specific memory region may vary from the default if the programmer uses certain API calls.

Note

The page migration behavior summarized by the following tables represents the current, observable behavior. Said behavior will likely change in the near future.

HSA_XNACK=1 Automatic Page Migration Enabled

Allocator	Initial Physical Location	CPU Access after GPU First Touch	Default Behavior for GPU Access
System Allocator (malloc,new,allocate, etc)	CPU DDR4	Migrate to CPU DDR4 on touch	Migrate to GPU HBM on touch
hipMallocManaged	CPU DDR4	Migrate to CPU DDR4 on touch	Migrate to GPU HBM on touch
hipHostMalloc	CPU DDR4	Local read/write	Zero copy read/write over Infinity Fabric
hipMalloc	GPU HBM	Zero copy read/write over Inifinity Fabric	Local read/write

Disabling XNACK will not necessarily result in an application failure, as most types of memory can still be accessed by the AMD “Optimized 3rd Gen EPYC” CPU and AMD MI250X GPU. In most cases, however, the access will occur in a zero-copy fashion over the Infinity Fabric. The exception is memory allocated through standard system allocators such as malloc, which cannot be accessed directly from GPU kernels without previously being registered via a HIP runtime call such as hipHostRegister. Access to malloc’ed and unregistered memory from GPU kernels will result in fatal unhandled page faults. The table below shows how common allocators behave with XNACK disabled.

HSA_XNACK=0 Automatic Page Migration Disabled

Allocator	Initial Physical Location	Default Behavior for CPU Access	Default Behavior for GPU Access
System Allocator (malloc,new,allocate, etc)	CPU DDR4	Local read/write	Fatal Unhandled Page Fault
hipMallocManaged	CPU DDR4	Zero copy read/write over Infinity Fabric	Local read/write
hipHostMalloc	CPU DDR4	Local read/write	Zero copy read/write over Infinity Fabric
hipMalloc	GPU HBM	Zero copy read/write over Inifinity Fabric	Local read/write

Compiling HIP kernels for specific XNACK modes

Although XNACK is a capability of the MI250X GPU, it does require that kernels be able to recover from page faults. Both the ROCm and CCE HIP compilers will default to generating code that runs correctly with both XNACK enabled and disabled. Some applications may benefit from using the following compilation options to target specific XNACK modes.

hipcc --amdgpu-target=gfx90a or CC --offload-arch=gfx90a -x hip

Kernels are compiled to a single “xnack any” binary, which will run correctly with both XNACK enabled and XNACK disabled.

hipcc --amdgpu-target=gfx90a:xnack+ or CC --offload-arch=gfx90a:xnack+ -x hip

Kernels are compiled in “xnack plus” mode and will only be able to run on GPUs with HSA_XNACK=1 to enable XNACK. Performance may be better than “xnack any”, but attempts to run with XNACK disabled will fail.

hipcc --amdgpu-target=gfx90a:xnack- or CC --offload-arch=gfx90a:xnack- -x hip

Kernels are compiled in “xnack minus” mode and will only be able to run on GPUs with HSA_XNACK=0 and XNACK disabled. Performance may be better than “xnack any”, but attempts to run with XNACK enabled will fail.

hipcc --amdgpu-target=gfx90a:xnack- --amdgpu-target=gfx90a:xnack+ -x hip or CC --offload-arch=gfx90a:xnack- --offload-arch=gfx90a:xnack+ -x hip

Two versions of each kernel will be generated, one that runs with XNACK disabled and one that runs if XNACK is enabled. This is different from “xnack any” in that two versions of each kernel are compiled and HIP picks the appropriate one at runtime, rather than there being a single version compatible with both. A “fat binary” compiled in this way will have the same performance of “xnack+” with HSA_XNACK=1 and as “xnack-” with HSA_XNACK=0, but the final executable will be larger since it contains two copies of every kernel.

If the HIP runtime cannot find a kernel image that matches the XNACK mode of the device, it will fail with hipErrorNoBinaryForGpu.

$ HSA_XNACK=0 srun -n 1 -N 1 -t 1 ./xnack_plus.exe
"hipErrorNoBinaryForGpu: Unable to find code object for all current devices!"
srun: error: frontier002: task 0: Aborted
srun: launch/slurm: _step_signal: Terminating StepId=74100.0

One way to diagnose hipErrorNoBinaryForGpu messages is to set the environment variable AMD_LOG_LEVEL to 1 or greater:

$ AMD_LOG_LEVEL=1 HSA_XNACK=0 srun -n 1 -N 1 -t 1 ./xnack_plus.exe
:1:rocdevice.cpp            :1573: 43966598070 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:rocdevice.cpp            :1573: 43966598762 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:rocdevice.cpp            :1573: 43966599392 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:rocdevice.cpp            :1573: 43966599970 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:rocdevice.cpp            :1573: 43966600550 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:rocdevice.cpp            :1573: 43966601109 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:rocdevice.cpp            :1573: 43966601673 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:rocdevice.cpp            :1573: 43966602248 us: HSA_AMD_AGENT_INFO_SVM_DIRECT_HOST_ACCESS query failed.
:1:hip_code_object.cpp      :460 : 43966602806 us: hipErrorNoBinaryForGpu: Unable to find code object for all current devices!
:1:hip_code_object.cpp      :461 : 43966602810 us:   Devices:
:1:hip_code_object.cpp      :464 : 43966602811 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :464 : 43966602811 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :464 : 43966602812 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :464 : 43966602813 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :464 : 43966602813 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :464 : 43966602814 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :464 : 43966602814 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :464 : 43966602815 us:     amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack- - [Not Found]
:1:hip_code_object.cpp      :468 : 43966602816 us:   Bundled Code Objects:
:1:hip_code_object.cpp      :485 : 43966602817 us:     host-x86_64-unknown-linux - [Unsupported]
:1:hip_code_object.cpp      :483 : 43966602818 us:     hipv4-amdgcn-amd-amdhsa--gfx90a:xnack+ - [code object v4 is amdgcn-amd-amdhsa--gfx90a:xnack+]
"hipErrorNoBinaryForGpu: Unable to find code object for all current devices!"
srun: error: frontier129: task 0: Aborted
srun: launch/slurm: _step_signal: Terminating StepId=74102.0

The above log messages indicate the type of image required by each device, given its current mode (amdgcn-amd-amdhsa--gfx90a:sramecc+:xnack-) and the images found in the binary (hipv4-amdgcn-amd-amdhsa--gfx90a:xnack+).

---

Floating-Point (FP) Atomic Operations and Coarse/Fine Grained Memory Allocations

The Frontier system, equipped with CDNA2-based architecture MI250X cards, offers a coherent host interface that enables advanced memory and unique cache coherency capabilities. The AMD driver leverages the Heterogeneous Memory Management (HMM) support in the Linux kernel to perform seamless page migrations to/from CPU/GPUs. This new capability comes with a memory model that needs to be understood completely to avoid unexpected behavior in real applications. For more details, please visit the previous section.

AMD GPUs can allocate two different types of memory locations: 1) Coarse grained and 2) Fine grained.

Coarse grained memory is only guaranteed to be coherent outside of GPU kernels that modify it, enabling higher performance memory operations. Changes applied to coarse-grained memory by a GPU kernel are only visible to the rest of the system (CPU or other GPUs) when the kernel has completed. A GPU kernel is only guaranteed to see changes applied to coarse grained memory by the rest of the system (CPU or other GPUs) if those changes were made before the kernel launched.

Fine grained memory allows CPUs and GPUs to synchronize (via atomics) and coherently communicate with each other while the GPU kernel is running, allowing more advanced programming patterns. The additional visibility impacts the performance of fine grained allocated memory.

The fast hardware-based Floating point (FP) atomic operations available on MI250X are assumed to be working on coarse grained memory regions; when these instructions are applied to a fine-grained memory region, they will silently produce a no-op. To avoid returning incorrect results, the compiler never emits hardware-based FP atomics instructions by default, even when applied to coarse grained memory regions. Currently, users can use the -munsafe-fp-atomics flag to force the compiler to emit hardware-based FP atomics. Using hardware-based FP atomics translates in a substantial performance improvement over the default choice.

Users applying floating point atomic operations (e.g., atomicAdd) on memory regions allocated via regular hipMalloc() can safely apply the -munsafe-fp-atomics flags to their codes to get the best possible performance and leverage hardware supported floating point atomics. Atomic operations supported in hardware on non-FP datatypes (e.g., INT32) will work correctly regardless of the nature of the memory region used.

In ROCm-5.1 and earlier versions, the flag -munsafe-fp-atomics is interpreted as a suggestion by the compiler, whereas from ROCm-5.2 the flag will always enforce the use of fast hardware-based FP atomics.

The following tables summarize the result granularity of various combinations of allocators, flags and arguments.

For hipHostMalloc(), the following table shows the nature of the memory returned based on the flag passed as argument.

API	Flag	Results
hipHostMalloc()	hipHostMallocDefault	Fine grained
hipHostMalloc()	hipHostMallocNonCoherent	Coarse grained

The following table shows the nature of the memory returned based on the flag passed as argument to hipExtMallocWithFlags().

API	Flag	Result
hipExtMallocWithFlags()	hipDeviceMallocDefault	Coarse grained
hipExtMallocWithFlags()	hipDeviceMallocFinegrained	Fine grained

Finally, the following table summarizes the nature of the memory returned based on the flag passed as argument to hipMallocManaged() and the use of CPU regular malloc() routine with the possible use of hipMemAdvise().

API	MemAdvice	Result
hipMallocManaged()		Fine grained
hipMallocManaged()	hipMemAdvise (hipMemAdviseSetCoarseGrain)	Coarse grained
malloc()		Fine grained
malloc()	hipMemAdvise (hipMemAdviseSetCoarseGrain)	Coarse grained

Performance considerations for LDS FP atomicAdd()

Hardware FP atomic operations performed in LDS memory are usually always faster than an equivalent CAS loop, in particular when contention on LDS memory locations is high. Because of a hardware design choice, FP32 LDS atomicAdd() operations can be slower than equivalent FP64 LDS atomicAdd(), in particular when contention on memory locations is low (e.g. random access pattern). The aforementioned behavior is only true for FP atomicAdd() operations. Hardware atomic operations for CAS/Min/Max on FP32 are usually faster than the FP64 counterparts. In cases when contention is very low, a FP32 CAS loop implementing an atomicAdd() operation could be faster than an hardware FP32 LDS atomicAdd(). Applications using single precision FP atomicAdd() are encouraged to experiment with the use of double precision to evaluate the trade-off between high atomicAdd() performance vs. potential lower occupancy due to higher LDS usage.

---

System Updates

2023-05-09

On Tuesday, May 9, 2023, the darshan-runtime modulefile was added to DefApps and is now loaded by default on Frontier. This module will allow users to profile the I/O of their applications with minimal impact. The logs are available to users on the Orion file system in /lustre/orion/darshan/<system>/<yyyy>/<mm>/<dd>. Unloading darshan-runtime is recommended for users profiling their applications with other profilers to prevent conflicts.

---

Known Issues

Open Issues

Running

OLCFDEV-1684: libfabric CXI provider fails to init in single-node runs on Frontier

It was reported to us that single-node jobsteps cannot initialize the libfabric CXI provider.

FATAL ERROR (proc 1): in gasnetc_ofi_init() at[..]/gasnet_ofi.c:1164: fi_domain failed: -38(Function not implemented)

This issue has been fixed in Slurm 23.02 which we are currently testing and expect to deploy on the system as soon as fully verified.

OLCFDEV-1510: MPI_Comm_spawn function and server not working

The MPI_Comm_spawn function within mpich and the spawn server for multi-node spawning are not working on Crusher.

Error reported:

MPICH ERROR [Rank 0] [job id 255254.0] [Thu Jan 26 17:41:16 2023] [crusher189] - Abort(403262725) (rank 0 in comm 0): Fatal error in PMPI_Comm_spawn: Invalid communicator, error stack:
PMPI_Comm_spawn(140): MPI_Comm_spawn(cmd="child", argv=(nil), maxprocs=4, MPI_INFO_NULL, root=0, MPI_COMM_NULL, intercomm=0x7fffffff690c, errors=(nil)) failed
PMPI_Comm_spawn(80).: Null communicator

Open Issues w/Workaround

Running

OLCFDEV-1657: rocSOLVER incorrect answer at large matrix sizes

A bug was identified in the GETRF routines in rocSOLVER that can produce incorrect answers in large matrix sizes. This affects matrix sizes greater than 23300 when using an 8-byte datatype (ie, {{double}}). For 16-byte complex datatypes, this affects matrix sizes greater than 16448.

Workaround:

We recommend users use HPE/Cray Libsci or MAGMA. A patch is in development for this issue, and this post will be updated when the patch is available. This issue is resolved in ROCm/5.5.1.

OLCFDEV-1655: Occasional seg-fault during MPI_Init

Occasionally, some applications may encounter a segmentation fault during MPI_Init. This is sometimes accompanied by messages about pmi_allgather, MPICH, or CXI failures. Some examples of this failure’s signature in stdout/stderr:

srun: error: frontier00572: task 1918: Segmentation fault (core dumped)
srun: launch/slurm: _step_signal: Terminating StepId=1291836.0
slurmstepd: error: *** STEP 1291836.0 ON frontier00001 CANCELLED AT 2023-03-31T04:21:06 ***

or

srun: error: frontier00175: task 314: Segmentation fault
srun: launch/slurm: _step_signal: Terminating StepId=1291835.0
slurmstepd: error: *** STEP 1291835.0 ON frontier00001 CANCELLED AT 2023-03-31T04:18:45 ***
Fri Mar 31 04:18:45 2023: `PE_9168]:inet_recv:inet_recv: unexpected EOF Success
Fri Mar 31 04:18:45 2023: [PE_9168]:_pmi_network_allgather:_pmi_inet_recv from controller failed
Fri Mar 31 04:18:45 2023: [PE_9168]:_pmi_allgather:_pmi_network_allgather failed...
slurmstepd: error: Failed to destroy CXI Service ID 5 (cxi2): -16
slurmstepd: error: Failed to destroy CXI Service ID 5 (cxi2): -16
slurmstepd: error: switch_g_job_postfini: Device or resource busy

or

Fri Mar 31 05:03:19 2023: [PE_552]:inet_recv:inet_recv: recv error (fd=4) Connection reset by peer
Fri Mar 31 05:03:19 2023: [PE_552]:_pmi_network_allgather:_pmi_inet_recv from controller failed
Fri Mar 31 05:03:19 2023: [PE_552]:_pmi_allgather:_pmi_network_allgather failed
Fri Mar 31 05:03:19 2023: [PE_576]:inet_recv:inet_recv: recv error (fd=4) Connection reset by peer
Fri Mar 31 05:03:19 2023: [PE_576]:_pmi_network_allgather:_pmi_inet_recv from controller failed
Fri Mar 31 05:03:19 2023: [PE_576]:_pmi_allgather:_pmi_network_allgather failed
MPICH ERROR [Rank 0] [job id 1291841.0] [Fri Mar 31 05:03:19 2023] [frontier00073] - Abort(1616271) (rank 0 in comm 0): Fatal error in PMPI_Init: Other MPI error, error stack:
MPIR_Init_thread(170).......:
MPID_Init(501)..............:
MPIDI_OFI_mpi_init_hook(805):
MPIDU_bc_table_create(204)..:  PMI_Allgather failed: -1aborting job:
Fatal error in PMPI_Init: Other MPI error, error stack:
MPIR_Init_thread(170).......:
MPID_Init(501)..............:
MPIDI_OFI_mpi_init_hook(805):
MPIDU_bc_table_create(204)..:  PMI_Allgather failed: -1
Fri Mar 31 05:03:19 2023: [PE_560]:inet_recv:inet_recv: recv error (fd=4) Connection reset by peer
Fri Mar 31 05:03:19 2023: [PE_560]:_pmi_network_allgather:_pmi_inet_recv from controller failed
Fri Mar 31 05:03:19 2023: [PE_560]:_pmi_allgather:_pmi_network_allgather failed
MPICH ERROR [Rank 0] [job id 1291841.0] [Fri Mar 31 05:03:19 2023] [frontier00071] - Abort(1616271) (rank 0 in comm 0): Fatal error in PMPI_Init: Other MPI error, error stack:

To identify this failure mode, load the core file and application into gdb (note: if you set ulimit -c 0, you will not get a corefile):

$ gdb /path/to/app/${my_exe} /path/to/corefile/core
...
(gdb) info threads
  Id   Target Id                         Frame
* 1    Thread 0x7fc370bff700 (LWP 60725) 0x00007fc49ac6d149 in munmap () from /lib64/libc.so.6
  2    Thread 0x7fc4a0362dc0 (LWP 60702) warning: Section `.reg-xstate/60702' in core file too small.
0x00007fc49ac58527 in sched_yield () from /lib64/libc.so.6
  3    Thread 0x7fc48327f700 (LWP 60724) warning: Section `.reg-xstate/60724' in core file too small.
0x00007fc49ac68c27 in ioctl () from /lib64/libc.so.6

(gdb) backtrace
#0  0x00007fc49ac6d149 in munmap () from /lib64/libc.so.6
#1  0x00007fc49abf4d07 in new_heap () from /lib64/libc.so.6
#2  0x00007fc49abf590b in arena_get2.part () from /lib64/libc.so.6
#3  0x00007fc49abf86c9 in tcache_init.part () from /lib64/libc.so.6
#4  0x00007fc49abf9bfe in free () from /lib64/libc.so.6
#5  0x00007fc49b02da3c in ?? () from /usr/lib64/libstdc++.so.6
#6  0x00007fc49feeb6ea in start_thread () from /lib64/libpthread.so.0
#7  0x00007fc49ac73a6f in clone () from /lib64/libc.so.6

(gdb) thread 2
[Switching to thread 2 (Thread 0x7fc4a0362dc0 (LWP 60702))]
#0  0x00007fc49ac58527 in sched_yield () from /lib64/libc.so.6
(gdb) bt
#0  0x00007fc49ac58527 in sched_yield () from /lib64/libc.so.6
#1  0x00007fc49f71cf55 in Init_shm_barrier () from /opt/cray/pe/lib64/libmpi_amd.so.12
#2  0x00007fc49f71c6f5 in MPIDU_Init_shm_alloc () from /opt/cray/pe/lib64/libmpi_amd.so.12
#3  0x00007fc49f71e1cd in MPIDU_bc_table_create () from /opt/cray/pe/lib64/libmpi_amd.so.12
#4  0x00007fc49f46e251 in MPIDI_OFI_mpi_init_hook () from /opt/cray/pe/lib64/libmpi_amd.so.12
#5  0x00007fc49f2f1ef6 in MPID_Init () from /opt/cray/pe/lib64/libmpi_amd.so.12
#6  0x00007fc49d9a3ce5 in MPIR_Init_thread () from /opt/cray/pe/lib64/libmpi_amd.so.12
#7  0x00007fc49d9a3ab4 in PMPI_Init () from /opt/cray/pe/lib64/libmpi_amd.so.12
...
#9  0x00000000007d903f in main ()

Thread 2 called sched_yield, which is why it is not the first thread loaded by GDB. However, this is the thread we are interested in, as it has the call to main(). Some variation on the stack trace after Line #7 (PMPI_INIT ()) may be expected.

If you encounter this failure, please let [help@olcf.ornl.gov <mailto:help@olcf.ornl.gov>`__ know.

Workaround:

Add #include <hip/hip_runtime.h> to the top the source file containing main, and run hipInit(0); prior to MPI_Init(&argc, &argv);. This file will now need to be compiled using the instructions in https://docs.olcf.ornl.gov/systems/frontier_user_guide.html#id9.

OLCFDEV-1479: rocFFT default cache file is in NFS shared directory - scalability issue

This behavior is fixed in ROCm/5.4.0. It affects all previous versions of ROCm.

By default rocFFT uses a location in $HOME to store its cache file. This is highly unsuitable for parallel runs with many processes all processes to try access the same file in $HOME, which is NFS share in our systems. This triggers a segfault when running more than 512 processes on Frontier/Crusher.

Workaround:

The following line forces rocFFT to use in-memory cache file:

export ROCFFT_RTC_CACHE_PATH=/dev/null

OLCFDEV-1597: OFI Poll Failed UNDELIVERABLE Errors

Fatal errors may be encountered when using MPI:

MPICH ERROR [Rank 1191] [job id 1275730.0] [Wed Feb 22 19:00:56 2023] [frontier04935] - Abort(271200399) (rank 1191 in comm 0): Fatal error in PMPI_Waitall: Other MPI error, error stack:
PMPI_Waitall(339)..............: MPI_Waitall(count=246, req_array=0x27350e0, status_array=0x3f08800) failed
MPIR_Waitall(167)..............:
MPIR_Waitall_impl(51)..........:
MPID_Progress_wait(193)........:
MPIDI_Progress_test(89)........:
MPIDI_OFI_handle_cq_error(1062): OFI poll failed (ofi_events.c:1064:MPIDI_OFI_handle_cq_error:Input/output error - UNDELIVERABLE)

Workaround:

Set the method used for on-node transfer with XPMEM to NONE:

export MPICH_SMP_SINGLE_COPY_MODE=NONE

OLCFDEV-1061: No GPU devices in ROCR_VISIBLE_DEVICES

Launching job steps without specifying an explicit GPU option via Slurm results leaves ROCR_VISIBLE_DEVICES unset.

$ srun -n 2 -c 1 ./hello_jobstep | sort
MPI 000 - OMP 000 - HWT 000 - Node crusher125 - RT_GPU_ID 0,1,2,3,4,5,6,7 - GPU_ID N/A - Bus_ID c1,c6,c9,ce,d1,d6,d9,de
MPI 001 - OMP 000 - HWT 008 - Node crusher125 - RT_GPU_ID 0,1,2,3,4,5,6,7 - GPU_ID N/A - Bus_ID c1,c6,c9,ce,d1,d6,d9,de

$  srun -n 2 -c 1 env | grep ROCR

Workaround

Instead, for example, using –ntasks-per-gpu or alternative GPU options will produce the expected behavior and set ROCR_VISIBLE_DEVICES to the correct values:

$ srun -n 2 -c 1 --ntasks-per-gpu=1 ./hello_jobstep | sort MPI 000 - OMP 000 - HWT 048 - Node crusher125 - RT_GPU_ID 0 - GPU_ID 0 - Bus_ID c1 MPI 001 - OMP 000 - HWT 056 - Node crusher125 - RT_GPU_ID 0 - GPU_ID 1 - Bus_ID c6

$ srun -n 2 -c 1 --ntasks-per-gpu=1 env | grep ROCR ROCR_VISIBLE_DEVICES=0 ROCR_VISIBLE_DEVICES=1

This is a change in Slurm 22.05 that was deployed on Crusher on July 5, 2022.

OLCFDEV-1485: Slurm “Unspecified error” when tasks per node is greater than 32

When running greater than 32 tasks per node, the following error can occur:

$ srun --ntasks-per-node=63 -n 126 -N 2 --gpus-per-node=8 --gpu-bind=closest ./hello_jobstep
srun: error: Task launch for StepId=136281.21 failed on node crusher001: Unspecified error
srun: error: Task launch for StepId=136281.21 failed on node crusher002: Unspecified error
srun: error: Application launch failed: Unspecified error
srun: Job step aborted

Workaround

The first thing to try would be to change your distribution flag to -m *:fcyclic or -m *:block. If the number of tasks per node is less than 64, then try setting -c 1 instead. If it is equal to 64 per node, then you can try combining those settings and set -c 1 -m *:fcyclic or -c 1 -m *:block.

This is a new behavior in Slurm 22.05 which was installed on July 5, 2022.

Resolved Issues