Dfyjkt

For asynchronous communication in MPI which of the following is better (in terms of performance, reliability, readability, etc.):

• MPI_Isend with buffer and then MPI_Iprobe & MPI_Recv once the receiver is ready, or


• MPI_Irecv with buffer (such that there is always and MPI_Irecv posted with sufficient buffer) and then MPI_Send when the sender is ready?

The communication scenario is that data has to be exchanged asynchronously and the arrival times do not matter and both processes have workload. Only the overall performance (especially no blocking) is considered.

Below is a minimal working example (I did not include workload and the timings are thus probably not meaningful).

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char const *argv) 
 MPI_Init(NULL, NULL);
 int world_size, world_rank;

 MPI_Comm_size(MPI_COMM_WORLD, &world_size);
 MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

 if (world_rank == 0 && world_size != 2) 
 fprintf(stderr, "This example requires two MPI processes.n");
 MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
 

 /* Non Blocking Send */
 int buf[100] = 0;

 MPI_Barrier(MPI_COMM_WORLD);
 double time = MPI_Wtime();
 if (world_rank == 1) 
 MPI_Request request;
 MPI_Isend(buf, 100, MPI_INT, 0, 0, MPI_COMM_WORLD, &request);
 MPI_Wait(&request, MPI_STATUS_IGNORE);
 else 
 MPI_Recv(buf, 100, MPI_INT, 1, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
 
 time = MPI_Wtime() - time;
 printf("rank = %d, time = %f secn", world_rank, time);
 MPI_Barrier(MPI_COMM_WORLD);

 usleep(100);
 if (world_rank == 0) 
 printf("---n");
 

 /* Non Blocking Receive */
 MPI_Barrier(MPI_COMM_WORLD);
 time = MPI_Wtime();
 if (world_rank == 1) 
 MPI_Send(buf, 100, MPI_INT, 0, 0, MPI_COMM_WORLD);
 else 
 MPI_Request request;
 MPI_Irecv(buf, 100, MPI_INT, 1, 0, MPI_COMM_WORLD, &request);
 MPI_Wait(&request, MPI_STATUS_IGNORE);
 
 time = MPI_Wtime() - time;
 printf("rank = %d, time = %f secn", world_rank, time);
 MPI_Barrier(MPI_COMM_WORLD);

 MPI_Finalize();
 return 0;

On my machine this generates:

rank = 0, time = 0.000035 sec
rank = 1, time = 0.000036 sec
---
rank = 0, time = 0.000035 sec
rank = 1, time = 0.000026 sec

Thank you already for your answers and have a nice day :)