[mpich-discuss] Isend and Recv

[Zhen Wang]

Hi MPIers,

I have some questions regarding MPI_Isend() and MPI_Recv(). MPICH-3.1.3 is
used to compile and run the attached code on Red Hat Enterprise Linux 6.3
(a shared memory machine). While n = 9999, the MPI_Recv() finishes
immediately after MPI_Isend(): (This is what I understand and expect)

MPI 1: Recv started at 09:53:53.
MPI 0: Isend started at 09:53:53.
MPI 1: Recv finished at 09:53:53.
MPI 0: Isend finished at 09:53:58.

When n = 99999, I get the following. The MPI_Recv() finishes after
MPI_Wait():

MPI 1: Recv started at 09:47:56.
MPI 0: Isend started at 09:47:56.
MPI 0: Isend finished at 09:48:01.
MPI 1: Recv finished at 09:48:01.

But with OpenMPI 1.8 and n = 99999, MPI_Recv() finishes immediately after
MPI_Isend():

MPI 0: Isend started at 09:55:28.
MPI 1: Recv started at 09:55:28.
MPI 1: Recv finished at 09:55:28.
MPI 0: Isend finished at 09:55:33.

Am I misunderstanding something here? In case the attached code is dropped,
the code is included. Thanks in advance.


#include "mpi.h"
#include <unistd.h>
#include <stdio.h>
#include "vector"
#include <time.h>

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

  int rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  int n = 9999;
  std::vector<int> vec(n);
  MPI_Request mpiRequest;
  MPI_Status mpiStatus;
  char tt[9] = {0};

  MPI_Barrier(MPI_COMM_WORLD);

  if (rank == 0)
  {
    MPI_Isend(&vec[0], n, MPI_INT, 1, 0, MPI_COMM_WORLD, &mpiRequest);
    time_t t = time(0);
    strftime(tt, 9, "%H:%M:%S", localtime(&t));
    printf("MPI %d: Isend started at %s.\n", rank, tt);

    //int done = 0;
    //while (done == 0)
    //{
    //  MPI_Test(&mpiRequest, &done, &mpiStatus);
    //}
    sleep(5);
    MPI_Wait(&mpiRequest, &mpiStatus);

    t = time(0);
    strftime(tt, 9, "%H:%M:%S", localtime(&t));
    printf("MPI %d: Isend finished at %s.\n", rank, tt);
  }
  else
  {
    time_t t = time(0);
    strftime(tt, 9, "%H:%M:%S", localtime(&t));
    printf("MPI %d: Recv started at %s.\n", rank, tt);

    MPI_Recv(&vec[0], n, MPI_INT, 0, 0, MPI_COMM_WORLD, &mpiStatus);

    t = time(0);
    strftime(tt, 9, "%H:%M:%S", localtime(&t));
    printf("MPI %d: Recv finished at %s.\n", rank, tt);
  }

  MPI_Finalize();

  return 0;
}



Best regards,
Zhen
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