Rocky MPI Hello World

MPI Jobs on Rocky

Supported MPI Stack

We currently recommend and support:

• OpenMPI 4.1.6
• PMIx 4.2.6

OpenMPI 5+ will be supported with a future software upgrade

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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

module load OpenMPI/4.1.6-GCC-13.2.0
mpicc -O2 -o hello_mpi hello_mpi.c

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

#!/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/4.1.6-GCC-13.2.0

srun --mpi=pmix_v3 ./hello_mpi

# optionally use mpirun instead of srun
#mpirun -np "$SLURM_NTASKS" ./hello_mpi

Submit the job:

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

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Quick Reference

Action	Command
Load MPI module	module load OpenMPI/4.1.6-GCC-13.2.0
Recommended launch	srun --mpi=pmix_v3 ./my_program
Alternative launch	mpirun ./my_program
Avoid	srun --mpi=pmi2, srun --mpi=none, OpenMPI 5