CORAL Systems

The LLNL and ORNL CORAL systems Lassen, Sierra, and Summit are pre-exascale supercomputers built by IBM. They run a specialized software stack that requires additional components to integrate properly with Flux. These components are provided as Lmod modules on all three systems.

To setup your environment to use these modules on the LLNL systems Lassen and Sierra, run:

module use /usr/global/tools/flux/blueos_3_ppc64le_ib/modulefiles

If you are using the ORNL system Summit, run:

module use /sw/summit/modulefiles/ums/gen007flux/Core

Launching Flux

Launching Flux on CORAL systems requires a shim layer to provide PMI on top of the PMIx interface provided by the CORAL system launcher jsrun. PMI is a common interface for bootstrapping parallel applications like MPI, SHMEM, and Flux. To load this module along with our side-installed Flux, run:

module load pmi-shim flux

We also suggest that you launch Flux using jsrun with the following arguments:

PMIX_MCA_gds="^ds12,ds21" jsrun -a 1 -c ALL_CPUS -g ALL_GPUS -n ${NUM_NODES} --bind=none --smpiargs="-disable_gpu_hooks" flux start

The PMIX_MCA_gds environment variable works around a bug in OpenPMIx that causes a hang when using the PMI compatibility shim. The ${NUM_NODES} variable is the number of nodes that you want to launch the Flux instance across. The remaining arguments ensure that all on-node resources are available to Flux for scheduling.

Launching Spectrum MPI within Flux

If you want to run MPI applications compiled with Spectrum MPI under Flux, then two steps are required. First, load our copy of Spectrum MPI that includes a backported fix from OpenMPI:

module load spectrum-mpi/2019.06.24-flux


Future releases of Spectrum MPI will include this patch, making loading this module unnecessary.

Second, when you run a Spectrum MPI binary under flux, enable Flux’s Spectrum MPI plugin. From the CLI, this looks like:

flux mini run -o mpi=spectrum my_mpi_binary

From the Python API, this looks like:

#!/usr/bin/env python3

import os
import flux
from flux import job

fh = flux.Flux()

jobspec = job.JobspecV1.from_command(['my_mpi_binary'])
jobspec.environment = dict(os.environ)
jobspec.setattr_shell_option('mpi', 'spectrum')

jobid = job.submit(fh, jobspec)

Scheduling GPUs

On all systems, Flux relies on hwloc to auto-detect the on-node resources available for scheduling. The hwloc that Flux is linked against must be configured with --enable-cuda for Flux to be able to detect Nvidia GPUs.

You can test to see if your system default hwloc is CUDA-enabled with:

lstopo | grep CoProc

If no output is produced, then your hwloc is not CUDA-enabled.

If running on an LLNL CORAL system, you can load a CUDA-enabled hwloc with:

module load hwloc/1.11.10-cuda