MPI basic example doesn't work

Question

I am trying to understand what was meant by the example below:

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

int main(int argc, char *argv[]) {
  int numtasks, rank, dest, source, rc, count, tag = 1;
  char inmsg, outmsg = 'x';
  MPI_Status Stat;

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

  if (rank == 0) {
    dest = 1;
    source = 1;
    rc = MPI_Send(&outmsg, 1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
    rc = MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
  } else if (rank == 1) {
    dest = 0;
    source = 0;
    rc = MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
    rc = MPI_Send(&outmsg, 1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
  }

  rc = MPI_Get_count(&Stat, MPI_CHAR, &count);
  printf("Task %d: Received %d char(s) from task %d with tag %d \n",
         rank, count, Stat.MPI_SOURCE, Stat.MPI_TAG);

  MPI_Finalize();
}

Essentially copied from here: https://computing.llnl.gov/tutorials/mpi/

Compiled like this:

$ mpicc -o ./mpi_test ./mpi_test.c

Run like this:

$ ./mpi_test

Gives this error:

Fatal error in MPI_Send: Invalid rank, error stack:
MPI_Send(171): MPI_Send(buf=0x7fff787b4d93, count=1, MPI_CHAR, dest=1, tag=1, MPI_COMM_WORLD) failed
MPI_Send(97).: Invalid rank has value 1 but must be nonnegative and less than 1

To be honest, I don't understand why the example should have worked in the first place. Can you please enlighten me? Is the example wrong, or is the error expected? Perhaps my setup is wrong?

PS. Compiled with gcc 4.7 on Fedora 18 amd64

score 5 · Accepted Answer · answered Nov 08 '13 at 23:12

5

You have to run your program using mpirun as follows:

mpirun -np 2 mpi_test

answered Nov 08 '13 at 23:12

Dale Myers

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Ugh, this is definitely closer, but now there's a problem with `mpirun`, which... tries to ssh to my machine, but can't for whatever reason? – Nov 08 '13 at 23:21
The above runs fine on my machine? We will need more output (probably better suited to a separate question too). – Dale Myers Nov 08 '13 at 23:23
OK, I've asked it in another question: http://stackoverflow.com/questions/19870394/run-hydra-mpiexec-locally-gives-strange-ssh-error I understand it that I missed some setup step, but I don't know which. – Nov 08 '13 at 23:33

MPI basic example doesn't work

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