FAQ:
Running jobs under SLURM

The short answer is just to use mpirun as normal.

The longer answer is that Open MPI supports launching parallel jobs in two of the three methods that SLURM supports:

1. Launching via "salloc ...": supported (older versions of SLURM used "srun -A ...")
2. Launching via "sbatch ...": supported (older versions of SLURM used "srun -B ...")
3. Launching via "srun -n X my_mpi_application": not supported

Specifically, you need to launch Open MPI's mpirun in an interactive SLURM allocation (via the salloc command) or you need to submit a script to SLURM (via the sbatch command). Open MPI does not yet support "direct" launching of MPI executables via srun.

Open MPI automatically obtains both the list of hosts and how many processes to start on each host from SLURM directly. Hence, it is unnecessary to specify the --hostfile, --host, or -np options to mpirun. Open MPI will also use SLURM-native mechanisms to launch and kill processes ([rsh] and/or ssh are not required).

For example:

# Allocate a SLURM job with 4 nodes
shell$ salloc -N 4 sh
# Now run an Open MPI job on all the nodes allocated by SLURM
# (Note that you need to specify -np for the 1.0 and 1.1 series;
# the -np value is inferred directly from SLURM starting with the 
# v1.2 series)
shell$ mpirun my_mpi_application

This will run the 4 MPI processes on the nodes that were allocated by SLURM. Equivalently, you can do this:

# Allocate a SLURM job with 4 nodes and run your MPI application in it
shell$ salloc -N 4 mpirun my_mpi_aplication

Or, if submitting a script:

shell$ cat my_script.sh
#!/bin/sh
mpirun my_mpi_application
shell$ sbatch -N 4 my_script.sh
srun: jobid 1234 submitted
shell$

It is on the to-do list, yes. But it has not bubbled up high enough in priority to occur yet.