c - MPI I_Send/MPI_Irecv issue

Question

I am writing an MPI program that has the first instance working as a master, sending and receiving results from its workers.

The receive function does something like this:

struct result *check_for_message(void) {
...
  static unsigned int message_size;
  static char *buffer;
  static bool started_reception = false;
  static MPI_Request req;

  if (!started_reception) {
    MPI_Irecv(&message_size, 1, MPI_INT, MPI_ANY_SOURCE, SIZE_TAG,
              MPI_COMM_WORLD, &req);
    started_reception = true;
  } else {
    int flag = 0;
    MPI_Status status;

    MPI_Test(&req, &flag, &status);
    if (flag == 1) {
      started_reception = false;
      buffer = calloc(message_size + 1, sizeof(char));
      DIE_IF_NULL(buffer); // printf + MPI_Finalize + exit
      MPI_Request content_req;

      MPI_Irecv(buffer, MAX_MSG_SIZE, MPI_CHAR, status.MPI_SOURCE, CONTENT_TAG,
                MPI_COMM_WORLD, &content_req);
      MPI_Wait(&content_req, MPI_STATUS_IGNORE);

      ret = process_request(buffer);
      free(buffer);
    }
  }

...
}

The send function does something like this:

  MPI_Request size_req;
  MPI_Request content_req;

  MPI_Isend(&size, 1, MPI_INT, dest, SIZE_TAG, MPI_COMM_WORLD, &size_req);
  MPI_Wait(&size_req, MPI_STATUS_IGNORE);
  MPI_Isend(buf, size, MPI_CHAR, dest, CONTENT_TAG, MPI_COMM_WORLD,
            &content_req);
  MPI_Wait(&content_req, MPI_STATUS_IGNORE);

I noticed that if I remove the MPI_Wait in the sending function it often happens that the execution blocks or some sort of SIGNAL stops the execution of an instance(I can check the output but I think it was something about a free error SIGSEGV).

When I add the MPI_Wait it always seems to run perfectly. Could it be something related to the order in which the two sends perform? Aren't they supposed to be in order?

I run the program locally with -n 16 but have also tested with -n 128. The messages that I send are above 50 chars (90% of the time), some being even > 300 chars.

question from:https://stackoverflow.com/questions/65865257/mpi-i-send-mpi-irecv-issue

Answer 1

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