[mpich-discuss] Non-blocking Collectives

[Jeff Hammond]

How is what you see not consistent with "The usual barrier semantics
are enforced at the corresponding completion operation (test or
wait)..."?

I don't know what you mean by "expected output and timing" since I
don't share your interpretation of the MPI standard, but I believe
that the n=3 case is absolutely consistent with the semantics of
MPI_Ibarrier+MPI_Wait.

Jeff

On Wed, Jun 26, 2013 at 2:54 PM, Jiri Simsa <jsimsa at cs.cmu.edu> wrote:
> Hi,
>
> I have a question about the semantics of non-blocking collective
> communication. For example, let's consider MPI_Ibarrier. The MPI 3.0
> standard specifies that:
>
> "MPI_IBARRIER is a nonblocking version of MPI_BARRIER. By calling
> MPI_IBARRIER, a process notifies that it has reached the barrier. The call
> returns immediately, indepen- dent of whether other processes have called
> MPI_IBARRIER. The usual barrier semantics are enforced at the corresponding
> completion operation (test or wait), which in the intra- communicator case
> will complete only after all other processes in the communicator have called
> MPI_IBARRIER. In the intercommunicator case, it will complete when all
> processes in the remote group have called MPI_IBARRIER."
>
> My understanding of the standard is that that MPI_Wait(&request, &status),
> where request has been previously passed into MPI_Ibarrier, returns after
> all processes in the respective intra-communicator called MPI_Ibarrier.
> However, the mpich-3.0.4 library, seems to in some cases wait for all
> processes in the respective intra-communicator to call MPI_Wait. Here is an
> example that demonstrates this behavior:
>
> #include <mpi.h>
> #include <unistd.h>
>
> int main( int argc, char *argv[]) {
>   MPI_Request request;
>   MPI_Status status;
>   MPI_Init(&argc, &argv );
>   int myrank;
>   MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
>   if (myrank == 0) {
>     MPI_Ibarrier(MPI_COMM_WORLD, &request);
>     MPI_Wait(&request, &status);
>     printf("%d, Completed barrier.\n", myrank);
>   } else {
>     MPI_Ibarrier(MPI_COMM_WORLD, &request);
>     sleep(1);
>     MPI_Wait(&request, &status);
>     printf("%d, Completed barrier.\n", myrank);
>   }
>   MPI_Finalize();
>   return 0;
> }
>
> When executed with "mpiexec -n 2 ./example", I see the expected output and
> timing. However, when executed with "mpiexec -n 3 ./example", the call to
> MPI_Wait in process 0 returns only after the other processes wake up from
> sleep() and call MPI_Wait.
>
> Isn't this a violation of the standard?
>
> Best,
>
> --Jiri Simsa
>
>
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-- 
Jeff Hammond
jeff.science at gmail.com