Collectives

C | Fortran-2008 | Fortran-90

MPI_Allgatherv

Definition

MPI_Allgatherv is a variant of MPI_Allgather; it collects data from all processes in a given communicator and stores the data collected in the receive buffer of each process. Unlike MPI_Allgather however, MPI_Allgatherv allows the messages received to have different lengths and be stored at arbitrary locations in the receive buffer. MPI_Allgatherv is a collective operation; all processes in the communicator must invoke this routine. Other variants of MPI_Allgatherv are MPI_Gather, MPI_Gatherv and MPI_Allgather. Refer to MPI_Iallgatherv to see the non-blocking counterpart of MPI_Allgatherv.

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int MPI_Allgatherv(void* buffer_send,
                   int count_send,
                   MPI_Datatype datatype_send,
                   void* buffer_recv,
                   const int* counts_recv,
                   const int* displacements,
                   MPI_Datatype datatype_recv,
                   MPI_Comm communicator);

Parameters

buffer_send

The buffer containing the data to send. The “in place” option for intra-communicators is specified by passing MPI_IN_PLACE as the value of buffer_send at the root. In such a case, count_send and datatype_send are ignored, and the contribution of the root to the gathered vector is assumed to be already in the correct place in the receive buffer.

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.

counts_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.

displacements

An array containing the displacement to apply to the message received by each process. Displacements are expressed in number of elements, not bytes.

datatype_recv

The type of one receive buffer element.

communicator

The communicator in which the variable all gather takes place.

Return value

The error code returned from the variable all gather.

Example

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

/**
 * @brief Illustrates how to use the variable version of an all gather.
 * @details This application is meant to be run with 3 MPI processes. Every MPI
 * process begins with a value, each process will gather all these values and
 * print them. The example is designed to cover all cases:
 * - Different displacements
 * - Different receive counts
 * It can be visualised as follows:
 *
 * +-----------+ +-----------+ +-------------------+ 
 * | Process 0 | | Process 1 | |     Process 2     |
 * +-+-------+-+ +-+-------+-+ +-+-------+-------+-+
 *   | Value |     | Value |     | Value | Value |
 *   |  100  |     |  101  |     |  102  |  103  |
 *   +-------+     +-------+     +-------+-------+
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *   +-----+-----+-----+-----+-----+-----+-----+
 *   | 100 |  0  |  0  | 101 |  0  | 102 | 103 |
 *   +-----+-----+-----+-----+-----+-----+-----+
 *   |               Each process              |
 *   +-----------------------------------------+
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Get 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);

    // Define the receive counts
    int counts[3] = {1, 1, 2};

    // Define the displacements
    int displacements[3] = {0, 3, 5};

    // Buffer in which receive the data collected
    int buffer[7] = {0};

    // Buffer containing our data to send
    int* my_values;
    int my_values_count;

    switch(my_rank)
    {
        case 0:
        {
            // Define my values
            my_values_count = 1;
            my_values = (int*)malloc(sizeof(int) * my_values_count);
            *my_values = 100;
            printf("Value sent by process %d: %d.\n", my_rank, *my_values);
            break;
        }
        case 1:
        {
            // Define my values
            my_values_count = 1;
            my_values = (int*)malloc(sizeof(int) * my_values_count);
            *my_values = 101;
            printf("Value sent by process %d: %d.\n", my_rank, *my_values);
            break;
        }
        case 2:
        {
            // Define my values
            my_values_count = 2;
            my_values = (int*)malloc(sizeof(int) * my_values_count);
            my_values[0] = 102;
            my_values[1] = 103;
            printf("Values sent by process %d: %d and %d.\n", my_rank, my_values[0], my_values[1]);
            break;
        }
    }

    MPI_Allgatherv(my_values, my_values_count, MPI_INT, buffer, counts, displacements, MPI_INT, MPI_COMM_WORLD);

    printf("Values gathered in the buffer on process %d:", my_rank);
    for(int i = 0; i < 7; i++)
    {
        printf(" %d", buffer[i]);
    }
    printf("\n");
    free(my_values);

    MPI_Finalize();

    return EXIT_SUCCESS;
}