[mpiwg-p2p] Ordering of P2P messages in multithreaded applications

[Jeff Hammond]

A send of 0 must be issued by one of the two threads before buffer turns to 1. Messages must match in order at receiver and that will lead to “0\n1\n” being printed at the target.

Jeff

Sent from my iPhone

> On Nov 15, 2018, at 4:16 AM, Joachim Protze via mpiwg-p2p <mpiwg-p2p at lists.mpi-forum.org> wrote:
> 
> Hi all,
> 
> I have a question on the "Semantics of Point-to-Point Communication" in a multithreaded context.
> 
> For me the situation for the code below is not clear, especially with respect to the paragraph in MPI-3.1 p.41, l.10-17 :
> 
> 
> void test(int rank) {
>  int msg = 0;
>  if (rank == 0) {
> #pragma omp parallel num_threads(2)
> #pragma omp critical
>    {
>      MPI_Send(&msg, 1, MPI_INT, 1, 42, MPI_COMM_WORLD);
>      msg++;
>    }
>  } else if (rank == 1) {
>    MPI_Recv(&msg, 1, MPI_INT, 0, 42, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
>    printf("Received %i\n", msg);
>    MPI_Recv(&msg, 1, MPI_INT, 0, 42, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
>    printf("Received %i\n", msg);
>  }
> }
> 
> Two threads on the first process send a message, the first thread sends 0, the second thread send 1. From OpenMP semantics, the first send happens before the second send.
> 
> Is there a guarantee, that the other process receives the 0 first?
> 
> Thanks,
> Joachim
> 
> 
> -- 
> Dipl.-Inf. Joachim Protze
> 
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>