Open MPI¶

Loading Open MPI¶

Open MPI is installed for various compilers in the software module farm. A compiler must be loaded before you can load the corresponding openmpi. For example, if you load the default gcc through module load gcc, then you can see which openmpi modules are available under the gcc module via module avail openmpi:

[user@n0000 ~]$ module load gcc
[user@n0000 ~]$ module list

Currently Loaded Modules:
  1) gcc/11.4.0

[user@n0000 ~]$ module avail openmpi

-------------- /global/software/rocky-8.x86_64/modfiles/gcc/11.4.0 -----------
   openmpi/4.1.3    openmpi/4.1.6 (D)

After this, you can load the default openmpi/4.1.6 through module load openmpi or by specifying the version module load openmpi/4.1.6. If you want to load the non-default openmpi/4.1.3 module, then you must specify the version: module load openmpi/4.1.3:

[user@n0000 ~]$ module load openmpi
[user@n0000 ~]$ module list

Currently Loaded Modules:
  1) gcc/11.4.0   2) ucx/1.14.1   3) openmpi/4.1.6

Compiling MPI applications with Open MPI¶

Open MPI compiler wrappers mpicc, mpicxx, mpifort can be used to compile MPI applications. For hello world C/C++/Fortran examples:

Examples

CC++Fortran

mpicc -o helloc hello_world.c

mpicc is the MPI wrapper to the gcc C compiler.

mpicxx -o hellocxx hello_world.cpp

mpicxx is the MPI wrapper to the gcc C++ compiler.

mpifort -o hellofortran hello_world.f90

mpifort is the MPI wrapper to the gfortran compiler.

The gcc/openmpi compiled binaries can be launched directly through srun inside of a slrum job script.