OpenMPI MPI_Barrier problems

Question

I having some synchronization issues using the OpenMPI implementation of MPI_Barrier:

int rank;
int nprocs;

int rc = MPI_Init(&argc, &argv);

if(rc          


        

Answer 1

There is nothing wrong with MPI_Barrier().

As Jens mentioned, the reason why you are not seeing the output you expected is because stdout is buffered on each processes. There is no guarantee that prints from multiple processes will be displayed on the calling process in order. (If stdout from each process is be transferred to the main process for printing in real time, that will lead to lots of unnecessary communication!)

If you want to convince yourself that the barrier works, you could try writing to a file instead. Having multiple processes writing to a single file may lead to extra complications, so you could have each proc writing to one file, then after the barrier, swap the files they write to. For example:

    Proc-0           Proc-1
      |                 |
 f0.write(..)     f1.write(...) 
      |                 |
      x  ~~ barrier ~~  x
      |                 |
 f1.write(..)     f0.write(...) 
      |                 |
     END               END

Sample implementation:

#include "mpi.h"
#include 
#include 

int main(int argc, char **argv) {
    char filename[20];
    int rank, size;
    FILE *fp;

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

    if (rank < 2) { /* proc 0 and 1 only */ 
        sprintf(filename, "file_%d.out", rank);
        fp = fopen(filename, "w");
        fprintf(fp, "P%d: before Barrier\n", rank);
        fclose(fp);
    }

    MPI_Barrier(MPI_COMM_WORLD);

    if (rank < 2) { /* proc 0 and 1 only */ 
        sprintf(filename, "file_%d.out", (rank==0)?1:0 );
        fp = fopen(filename, "a");
        fprintf(fp, "P%d: after Barrier\n", rank);
        fclose(fp);
    }

    MPI_Finalize();
    return 0;

}

After running the code, you should get the following results:

[me@home]$ cat file_0.out
P0: before Barrier
P1: after Barrier

[me@home]$ cat file_1.out
P1: before Barrier
P0: after Barrier

For all files, the "after Barrier" statements will always appear later.