How to run this compiled open MPI program (C)?

Question

I am trying to run the example code at the following url. I compiled the program with "mpicc twoGroups.c" and tried to run it as "./a.out", but got the following message: Must specify MP_PROCS= 8. Terminating.

My question is how do you set MP_PROCS=8 ?

Group and Communication Routine examples at here https://computing.llnl.gov/tutorials/mpi/

#include "mpi.h"
#include <stdio.h>
#define NPROCS 8

main(int argc, char *argv[])  {
int        rank, new_rank, sendbuf, recvbuf, numtasks,
           ranks1[4]={0,1,2,3}, ranks2[4]={4,5,6,7};
MPI_Group  orig_group, new_group;
MPI_Comm   new_comm;

MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &numtasks);

if (numtasks != NPROCS) {
  printf("Must specify MP_PROCS= %d. Terminating.\n",NPROCS);
  MPI_Finalize();
  exit(0);
  }

sendbuf = rank;

/* Extract the original group handle */
MPI_Comm_group(MPI_COMM_WORLD, &orig_group);

/* Divide tasks into two distinct groups based upon rank */
if (rank < NPROCS/2) {
  MPI_Group_incl(orig_group, NPROCS/2, ranks1, &new_group);
  }
else {
  MPI_Group_incl(orig_group, NPROCS/2, ranks2, &new_group);
  }

/* Create new new communicator and then perform collective communications */
MPI_Comm_create(MPI_COMM_WORLD, new_group, &new_comm);
MPI_Allreduce(&sendbuf, &recvbuf, 1, MPI_INT, MPI_SUM, new_comm);

MPI_Group_rank (new_group, &new_rank);
printf("rank= %d newrank= %d recvbuf= %d\n",rank,new_rank,recvbuf);

MPI_Finalize();
}

Answer 1

When you execute an MPI program, you need to use the appropriate wrappers. Most of the time it looks like this:

mpiexec -n <number_of_processes> <executable_name> <executable_args>

So for your simple example:

mpiexec -n 8 ./a.out

You will also see mpirun used instead of mpiexec or -np instead of -n . Both are fine most of the time.

If you're just starting out, it would also be a good idea to make sure you're using a recent version of MPI so you don't get old bugs or weird execution environments. MPICH and Open MPI are the two most popular implementations. MPICH just released version 3.1 available here while Open MPI has version 1.7.4 available here . You can also usually get either of them via your friendly neighborhood package manager.