[mpich-discuss] Isend and Recv

Junchao Zhang jczhang at mcs.anl.gov
Wed Nov 5 13:28:28 CST 2014


I think it is because MPICH just crosses over the eager to rendezvous mode
threshold, when  n goes from 9999 to 99999.  OpenMPI certainly uses a
different threshold than MPICH.
When you install MPICH, a utility program mpivars is also installed. Type
'mpivars | grep EAGER',  you will get default values for various eager
thresholds.

In your case, export MPIR_CVAR_CH3_EAGER_MAX_MSG_SIZE=5000000 and you will
get the same result as OpenMPI.

--Junchao Zhang

On Wed, Nov 5, 2014 at 11:20 AM, Zhen Wang <toddwz at gmail.com> wrote:

> 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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