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mpinonblockingopenmpi

MPI non-blocking send and receive ordering

On a 2-core system, for the following code

if(rank == 0)
  {
    MPI_Isend(A) // to rank 1
    // Do something else.
    MPI_Isend(B) // to rank 1
    // Do something else.
    MPI_Wait(B is sent)
    MPI_Wait(A is sent)
  }
  else
  {
    MPI_Irecv(buffer1) // Listen to rank 0
    // Do something else.
    MPI_Irecv(buffer2) // Listen to rank 0
    // Do something else.
    MPI_Wait(buffer2 is finished receiving)
    MPI_Wait(buffer1 is finished receiving)
  }

Would rank 1 be guaranteed to receive A in buffer 1, and B in buffer 2?

Thank you!

Solution

  • MPI messages are "non-overtaking". Two messages from the same source can not arrive in a different order from how they are sent (blocking case) or initiated (non-blocking case). Of course, you can always set your mind at ease by specifying different tags.