c++ - Openmpi mpmd get communication size

Question

I have two openmpi programs which I start like this

mpirun -n 4 ./prog1 : -n 2 ./prog2

Now how do I use MPI_Comm_size(MPI_COMM_WORLD, &size) such that i get size values as

prog1 size=4
prog2 size=2.

As of now I get "6" in both programs.

Answer 1

This is doable albeit a bit cumbersome to get that. The principle is to split MPI_COMM_WORLD into communicators based on the value of argv[0], which contains the executable's name.

That could be something like that:

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

int main( int argc, char *argv[] ) {

    MPI_Init( &argc, &argv );

    int wRank, wSize;
    MPI_Comm_rank( MPI_COMM_WORLD, &wRank );
    MPI_Comm_size( MPI_COMM_WORLD, &wSize );

    int myLen = strlen( argv[0] ) + 1;
    int maxLen;
    // Gathering the maximum length of the executable' name
    MPI_Allreduce( &myLen, &maxLen, 1, MPI_INT, MPI_MAX, MPI_COMM_WORLD );

    // Allocating memory for all of them
    char *names = malloc( wSize * maxLen );
    // and copying my name at its place in the array
    strcpy( names + ( wRank * maxLen ), argv[0] );

    // Now collecting all executable' names
    MPI_Allgather( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL,
                   names, maxLen, MPI_CHAR, MPI_COMM_WORLD );

    // With that, I can sort-out who is executing the same binary as me
    int binIdx = 0;
    while( strcmp( argv[0], names + binIdx * maxLen ) != 0 ) {
        binIdx++;
    }
    free( names );

    // Now, all processes with the same binIdx value are running the same binary
    // I can split MPI_COMM_WORLD accordingly
    MPI_Comm binComm;
    MPI_Comm_split( MPI_COMM_WORLD, binIdx, wRank, &binComm );

    int bRank, bSize;
    MPI_Comm_rank( binComm, &bRank );
    MPI_Comm_size( binComm, &bSize );

    printf( "Hello from process WORLD %d/%d running %d/%d %s binary
",
            wRank, wSize, bRank, bSize, argv[0] );

    MPI_Comm_free( &binComm );

    MPI_Finalize();

    return 0;
}

On my machine, I compiled and ran it as follow:

~> mpicc mpmd.c
~> cp a.out b.out
~> mpirun -n 3 ./a.out : -n 2 ./b.out
Hello from process WORLD 0/5 running 0/3 ./a.out binary
Hello from process WORLD 1/5 running 1/3 ./a.out binary
Hello from process WORLD 4/5 running 1/2 ./b.out binary
Hello from process WORLD 2/5 running 2/3 ./a.out binary
Hello from process WORLD 3/5 running 0/2 ./b.out binary

Ideally, this could be greatly simplified by using MPI_Comm_split_type() if the corresponding type for sorting out by binaries existed. Unfortunately, there is no such MPI_COMM_TYPE_ pre-defined in the 3.1 MPI standard. The only pre-defined one is MPI_COMM_TYPE_SHARED to sort-out between processes running on the same shared memory compute nodes... Too bad! Maybe something to consider for the next version of the standard?