# 10.7. 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.

## 10.7.1. 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.

## 10.7.2. 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.

## 10.7.3. 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.