Synchronisation

C | Fortran-2008 | Fortran-90

MPI_Testany

Parameters

count

Number of request handlers in the array of requests passed.

requests

The request handles on the non-blocking routines to test.

index

The variable in which store the index of the request handler corresponding to the non-blocking routine that completed. If none is complete, it is set to MPI_UNDEFINED. Note that in C the index starts at 0.

flag

The variable in which store the check result; true if at least one of the underlying non-blocking operations are complete, false otherwise.

status

The variable in which store the status corresponding to the non-blocking routine that completed.

Return value

The error code returned from the request testings.

Example

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

/**
 * @brief Illustrates how to check multiple non-blocking routines for the 
 * completion of one of them.
 * @details This program is meant to be run with 3 processes: a sender and two
 * receivers. The sender emits 2 messages in a non-blocking fashion. It then
 * tests the corresponding request handlers to see which operation finished. It
 * then repeats the process to check the other operation to finish.
 * This application covers two cases:
 * - None of the underlying non-blocking operations is complete yet.
 * - One of the underlying non-blocking operations is complete.
 * 
 *                  +---------------+---------------+
 *                  | MPI_Recv from | MPI_Recv from |
 *                  | process 1 is  | process 2 is  |
 *                  |    complete   |    complete   |
 * +----------------+---------------+---------------+
 * | MPI_Testany #1 |               |               |
 * | MPI_Testany #2 |       X       |               |
 * | MPI_Testany #3 |               |       X       |
 * +----------------+---------------+---------------+
 *
 * (Note to the reader: the use of MPI_Barrier is only to control the execution
 * flow to make sure it exposes the different cases mentioned above when the
 * MPI_Testany are called.)
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Get the number of processes and check only 3 processes are used
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if(size != 3)
    {
        printf("This application is meant to be run with 3 processes.\n");
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

    // Get my rank
    int my_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    if(my_rank == 0)
    {
        // The "master" MPI process sends the messages.
        int buffer[2] = {12345, 67890};
        int count = 2;
        MPI_Request requests[2];
        int recipient_rank_of_request[2];

        // Send first message to process 1
        printf("[Process %d] Sends %d to process 1.\n", my_rank, buffer[0]);
        MPI_Issend(&buffer[0], 1, MPI_INT, 1, 0, MPI_COMM_WORLD, &requests[0]);
        recipient_rank_of_request[0] = 1;

        // Send second message to process 2
        printf("[Process %d] Sends %d to process 2.\n", my_rank, buffer[1]);
        MPI_Issend(&buffer[1], 1, MPI_INT, 2, 0, MPI_COMM_WORLD, &requests[1]);
        recipient_rank_of_request[1] = 2;

        int index;
        int ready;

        // Check if one of the requests finished
        MPI_Testany(count, requests, &index, &ready, MPI_STATUS_IGNORE);
        if(ready)
            printf("MPI_Issend to process %d completed.\n", index + 1);
        else
            printf("None of the MPI_Issend completed for now.\n");

        // Tell receivers they can now issue the first MPI_Recv.
        MPI_Barrier(MPI_COMM_WORLD);

        // Receivers tell us the MPI_Recv is complete.
        MPI_Barrier(MPI_COMM_WORLD);

        // Check if one of the requests finished
        MPI_Testany(count, requests, &index, &ready, MPI_STATUS_IGNORE);
        if(ready)
            printf("MPI_Issend to process %d completed.\n", recipient_rank_of_request[index]);
        else
            printf("None of the MPI_Issend completed for now.\n");

        // Tell receivers they can now issue the second MPI_Recv.
        MPI_Barrier(MPI_COMM_WORLD);

        // Receivers tell us the second MPI_Recv is complete.
        MPI_Barrier(MPI_COMM_WORLD);

        // Check if one of the requests finished
        MPI_Testany(count, requests, &index, &ready, MPI_STATUS_IGNORE);
        if(ready)
            printf("MPI_Issend to process %d completed.\n", recipient_rank_of_request[index]);
        else
            printf("None of the MPI_Issend completed for now.\n");
    }
    else
    {
        // The "slave" MPI processes receive the messages.

        // Wait until the first MPI_Testany is issued.
        MPI_Barrier(MPI_COMM_WORLD);

        if(my_rank == 1)
        {
            int received;
            MPI_Recv(&received, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("[Process %d] Received value %d.\n", my_rank, received);
        }

        // Tell that the first MPI_Recv is issued.
        MPI_Barrier(MPI_COMM_WORLD);

        // Wait until the second MPI_Testany is issued.
        MPI_Barrier(MPI_COMM_WORLD);

        if(my_rank == 2)
        {
            int received;
            MPI_Recv(&received, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("[Process %d] Received value %d.\n", my_rank, received);
        }

        // Wait for the second MPI_Recv to be issued.
        MPI_Barrier(MPI_COMM_WORLD);
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}