Rocky MPI Hello World

MPI Jobs on Rocky

Supported versions

Rocky supports PMIx v3 for MPI process management. MPI applications should be launched via srun using --mpi=pmix_v3.

To see what versions of OpenMPI are available use the following command:

module spider OpenMPI

As of the writing of this document the versions available:

     Versions:
        OpenMPI/3.1.1-GCC-7.3.0-2.30
        OpenMPI/3.1.3-GCC-8.2.0-2.31.1
        OpenMPI/3.1.4-GCC-8.3.0
        OpenMPI/4.0.3-GCC-9.3.0
        OpenMPI/4.0.5-GCC-10.2.0
        OpenMPI/4.1.1-GCC-10.3.0
        OpenMPI/4.1.1-GCC-11.2.0
        OpenMPI/4.1.4-GCC-11.3.0
        OpenMPI/4.1.4-GCC-12.2.0
        OpenMPI/4.1.5-GCC-12.3.0
        OpenMPI/4.1.6-GCC-13.2.0
        OpenMPI/5.0.3-GCC-13.3.0
        OpenMPI/5.0.7-GCC-14.2.0
        OpenMPI/5.0.8-GCC-14.3.0
        OpenMPI/5.0.8-llvm-compilers-20.1.8

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MPI Example: Hello World in C

This example demonstrates compiling and running MPI across multiple nodes.

Step 1 — Create the MPI program (hello_mpi.c)

#include <mpi.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
    int rank, size, len;
    char name[MPI_MAX_PROCESSOR_NAME];

    MPI_Init(&argc, &argv);

    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    MPI_Get_processor_name(name, &len);

    printf("Hello from rank %d of %d on %s", rank, size, name);

    MPI_Finalize();
    return 0;
}

Step 2 — Compile the program

Load the appropriate version of OpenMPI you want to use and then compile your program with mpicc.

module load OpenMPI/5.0.8-GCC-14.3.0
mpicc -O2 -o hello_mpi hello_mpi.c

Step 3 — Submit an MPI batch job (mpi_hello.sbatch)

Create the batch file that we'll name mpi_hello.sbatch.
Notice that we are using --mpi=pmix_v3 as a parameter to srun.

#!/usr/bin/env bash
#SBATCH --job-name=mpi-hello
#SBATCH --output=mpi-hello-%j.out
#SBATCH --error=mpi-hello-%j.err
#SBATCH --nodes=2
#SBATCH --ntasks=8
#SBATCH --time=00:05:00

module load OpenMPI/5.0.8-GCC-14.3.0

srun --mpi=pmix_v3 ./hello_mpi

Use sbatch to submit the job defined in our sbatch file:

sbatch mpi_hello.sbatch

Step 4 — Expected Output

The generated output file (mpi-hello-<jobid>.out) should contain:

Hello from rank 0 of 8 on rocky4.rocky.nimbios.org
Hello from rank 1 of 8 on rocky4.rocky.nimbios.org
Hello from rank 2 of 8 on rocky4.rocky.nimbios.org
Hello from rank 3 of 8 on rocky4.rocky.nimbios.org
Hello from rank 4 of 8 on rocky5.rocky.nimbios.org
Hello from rank 5 of 8 on rocky5.rocky.nimbios.org
Hello from rank 6 of 8 on rocky5.rocky.nimbios.org
Hello from rank 7 of 8 on rocky5.rocky.nimbios.org