9.5. Launching with Slurm

Open MPI supports two modes of launching parallel MPI jobs under Slurm:

  1. Using Open MPI’s full-features mpirun launcher.

  2. Using Slurm’s “direct launch” capability.

Unless there is a strong reason to use srun for direct launch, the Open MPI team recommends using mpirun for launching under Slurm jobs.

Note

In versions of Open MPI prior to 5.0.x, using srun for direct launch could be faster than using mpirun. This is no longer true.

9.5.1. Using mpirun

When mpirun is launched in a Slurm job, mpirun will automatically utilize the Slurm infrastructure for launching and controlling the individual MPI processes. Hence, it is unnecessary to specify the --hostfile, --host, or -n options to mpirun.

Note

Using mpirun is the recommended method for launching Open MPI jobs in Slurm jobs.

mpirun’s Slurm support should always be available, regardless of how Open MPI or Slurm was installed.

For example:

# Allocate a Slurm job with 4 slots
shell$ salloc -n 4
salloc: Granted job allocation 1234

# Now run an Open MPI job on all the slots allocated by Slurm
shell$ mpirun mpi-hello-world

This will run the 4 MPI processes on the node(s) that were allocated by Slurm.

Or, if submitting a script:

shell$ cat my_script.sh
#!/bin/sh
mpirun mpi-hello-world
shell$ sbatch -n 4 my_script.sh
srun: jobid 1235 submitted
shell$

Similar to the salloc case, no command line options specifying number of MPI processes were necessary, since Open MPI will obtain that information directly from Slurm at run time.

9.5.2. Using Slurm’s “direct launch” functionality

Assuming that Slurm installed its Open MPI plugin, you can use srun to “direct launch” Open MPI applications without the use of Open MPI’s mpirun command.

Note

Using direct launch can be slightly faster when launching very, very large MPI processes (i.e., thousands or millions of MPI processes in a single job). But it has significantly fewer features than Open MPI’s mpirun.

First, you must ensure that Slurm was built and installed with PMI-2 support.

Note

Please ask your friendly neighborhood Slurm developer to support PMIx. PMIx is the current generation of run-time support API; PMI-2 is the legacy / antiquated API. Open MPI only supports PMI-2 for Slurm.

Next, ensure that Open MPI was configured --with-pmi=DIR, where DIR is the path to the directory where Slurm’s pmi2.h is located.

Open MPI applications can then be launched directly via the srun command. For example:

shell$ srun -N 4 mpi-hello-world

Or you can use sbatch with a script:

shell$ cat my_script.sh
#!/bin/sh
srun mpi-hello-world
shell$ sbatch -N 4 my_script.sh
srun: jobid 1235 submitted
shell$

Similar using mpirun inside of an sbatch batch script, no srun command line options specifying number of processes were necessary, because sbatch set all the relevant Slurm-level parameters about number of processes, cores, partition, etc.

9.5.3. Slurm 20.11

There were some changes in Slurm behavior that were introduced in Slurm 20.11.0 and subsequently reverted out in Slurm 20.11.3.

SchedMD (the makers of Slurm) strongly suggest that all Open MPI users avoid using Slurm versions 20.11.0 through 20.11.2.

Indeed, you will likely run into problems using just about any version of Open MPI these problematic Slurm releases.

Important

Please either downgrade to an older version or upgrade to a newer version of Slurm.