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MPI_Iallgather

Definition

MPI_Iallgather is the non-blocking version of MPI_Allgather; it collects data from all processes in a given communicator and stores the gathered data in the receive buffer of each process. Unlike MPI_Allgather however, it will not wait for the collection to complete and will return immediately instead. The user must therefore check for completion with MPI_Wait or MPI_Test before the buffers passed can be safely reused. MPI_Iallgather is a collective operation; all processes in the communicator must invoke this routine. Other variants of MPI_Iallgather are MPI_Igather, MPI_Igatherv and MPI_Iallgatherv. Refer to MPI_Allgather to see the blocking counterpart of MPI_Iallgather.

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int MPI_Iallgather(void* buffer_send,
                   int count_send,
                   MPI_Datatype datatype_send,
                   void* buffer_recv,
                   const int* count_recv,
                   const int* displacements,
                   MPI_Datatype datatype_recv,
                   MPI_Comm communicator,
                   MPI_Request* request);

Parameters

buffer_send
The buffer containing the data to send.
count_send
The number of elements in the send buffer.
datatype_send
The type of one send buffer element.
buffer_recv
The buffer in which store the gathered data for the root process. For other processes, the receiving parameters like this one are ignored.
count_recv
An array containing the number of elements in the message to receive from each process, not the total number of elements to receive from all processes altogether. For non-root processes, the receiving parameters like this one are ignored.
displacements
An array containing the displacement to apply to the message received by each process. Displacements are expressed in number of elements, not bytes. For non-root processes, the receiving parameters like this one are ignored.
datatype_recv
The type of one receive buffer element. For non-root processes, the receiving parameters like this one are ignored.
communicator
The communicator in which the gather takes place.
request
The variable in which store the handler on the non-blocking operation.

Returned value

MPI_SUCCESS
The routine successfully completed.

Example

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

/**
 * @brief Illustrates how to use a gather in a non-blocking way.
 * @details This application is meant to be run with 3 MPI processes. Every MPI
 * process begins with a value, then every MPI process collects the entirety of
 * the data gathered and moves on immediately to do something else while the
 * gather progresses. They then wait for the gather to complete before printing
 * the data gathered. It can be visualised as follows:
 *
 * +-----------+           +-----------+           +-----------+
 * | Process 0 |           | Process 1 |           | Process 2 |
 * +-+-------+-+           +-+-------+-+           +-+-------+-+
 *   | Value |               | Value |               | Value |
 *   |   0   |               |  100  |               |  200  |
 *   +-------+               +-------+               +-------+
 *     | | |  _________________| | |_________________  | | |
 *     | | |_|___________________|_____________      | | | |
 *     | |___|_____________      |             |     | | | |
 *     |     |      _______|_____|_____________|_____|_| | |
 *     |     |     |       |     |      _______|_____|___| |
 *     |     |     |       |     |     |       |     |     |
 *  +-----+-----+-----+ +-----+-----+-----+ +-----+-----+-----+
 *  |  0  | 100 | 200 | |  0  | 100 | 200 | |  0  | 100 | 200 |
 *  +-----+-----+-----+ +-----+-----+-----+ +-----+-----+-----+
 *  |    Process 0    | |    Process 1    | |    Process 2    |
 *  +-----------------+ +-----------------+ +-----------------+
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Get number of processes and check that 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 MPI processes.\n");
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

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

    // Define my value
    int my_value = my_rank * 100;
    printf("Process %d, my value = %d.\n", my_rank, my_value);

    // Issue the gather and move on immediately after, before the MPI_Iallgather completes
    int buffer[3];
    MPI_Request request;
    MPI_Iallgather(&my_value, 1, MPI_INT, buffer, 1, MPI_INT, MPI_COMM_WORLD, &request);

    // Do another job while the gather progresses
    // ...

    // Wait for the gather to complete before printing the values received
    MPI_Wait(&request, MPI_STATUS_IGNORE);
    printf("Values collected on process %d: %d, %d, %d.\n", my_rank, buffer[0], buffer[1], buffer[2]);

    MPI_Finalize();

    return EXIT_SUCCESS;
}