Create MPI processes on the fly with fork?

Question

If I use MPI, I have a number of processes specified when I run the main program. However I would like to start with one process and dynamically decide at runtime if and when I need more, to fork more processes off. Is that or something similar possible?

Otherwise I would have to reinvent MPI which I would very much like to avoid.

Answer 1

It is not possible to use fork() as the child process will not be able to use MPI functions. There is a simple mechanism in MPI to create dynamically new processes. You must use the MPI_Comm_spawn function or the MPI_Comm_spawn_mutliple

OpenMPI doc: http://www.open-mpi.org/doc/v1.4/man3/MPI_Comm_spawn.3.php

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

#define NUM_SPAWNS 2

int main( int argc, char *argv[] )
{
  int np = NUM_SPAWNS;
  int errcodes[NUM_SPAWNS];
  MPI_Comm parentcomm, intercomm;

  MPI_Init( &argc, &argv );
  MPI_Comm_get_parent( &parentcomm );
  if (parentcomm == MPI_COMM_NULL) {
    MPI_Comm_spawn( "spawn_example", MPI_ARGV_NULL, np, MPI_INFO_NULL, 0, MPI_COMM_WORLD, &intercomm, errcodes );
    printf("I'm the parent.\n");
  } else {
    printf("I'm the spawned.\n");
  }
  fflush(stdout);
  MPI_Finalize();
  return 0;
}