Open MPI¶

The Open MPI Project is an open source Message Passing Interface implementation that is developed and maintained by a consortium of academic, research, and industry partners. Open MPI is therefore able to combine the expertise, technologies, and resources from all across the High Performance Computing community in order to build the best MPI library available. Open MPI offers advantages for system and software vendors, application developers and computer science researchers.

This wiki is primarily intended for NERSC users who wish to use Open MPI on Perlmutter, however these instructions are sufficiently general that they should be largely applicable to other Cray EX systems running SLURM.

Using Open MPI at NERSC Perlmutter system¶

Compiling¶

Load the Open MPI module to pick up the packages compiler wrappers, mpirun launch command, and other utilities:

The following will load the default package

module use /global/common/software/m3169/perlmutter/modulefiles
module load openmpi

Open MPI is available for use with the following Cray programming environments:

• PrgEnv-gnu
• PrgEnv-cray
• PrgEnv-nvidia

Work is underway to provide a module for the LLVM programming environment.

The module file will detect which compiler environment you have loaded and load the appropriately built Open MPI package.

The simplest way to compile your application when using Open MPI is via the MPI compiler wrappers, e.g.

mpicc -o my_c_exec my_c_prog.c
mpif90 -o my_f90_exec my_f90_prog.f90

You pass extra compiler options to the back end compiler just as you would if using the compiler (not the cray wrappers) directly. Note by default the Open MPI compiler wrappers will build dynamic executables.

Job Launch¶

There are two ways to launch applications compiled against Open MPI on Perlmutter You can either use the Open MPI supplied mpirun job launcher, or Slurm's srun job launcher (termed native launch in some Open MPI documentation) , e.g.

salloc -N 6 --ntasks-per-node=32 -C haswell
srun --mpi=pmix -n 192 ./my_c_exec

or

salloc -N 6 --ntasks-per-node=32 -C haswell
mpirun -np 192 ./my_c_exec

For srun launched jobs, one can also use the SLURM environment variable SLURM_MPI_TYPE to set the mpi launch type. Open MPI 5.0.0 and newer only work with the pmix flavor.

If you wish to use srun, you should use the same srun options as if your application was compiled and linked against the vendor's MPI implementation.

(See also our running jobs example for Open MPI).

See the mpirun man page for more details about command line options. mpirun --help may also be used to get more information about mpirun command line options.

Note if you wish to use MPI dynamic process functionality such as MPI_Spawn, or MPI-4 Sessions related functions like MPI_Comm_create_from_group,
you must use mpirun to launch the application.

Using Java Applications with Open MPI¶

The Open MPI supports a Java interface. Note this interface has not been standardized by the MPI Forum. Information on how use Open MPI's Java interface is available on the Open MPI Java FAQ. Note you will need to load the java module to use Open MPI's Java interface.