Collectives

Reduction

C | Fortran-2008 | Fortran-90

MPI_Scan

Definition

MPI_Scan is an inclusive scan: it performs a prefix reduction across all MPI processes in the given communicator. In other words, each MPI process receives the result of the reduction operation on the values passed by that MPI process and all MPI processes with a lower rank. MPI_Scan is a collective operation; it must be called by all MPI processes in the communicator concerned. The variant of MPI_Scan is the exclusive version MPI_Exscan.

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int MPI_Scan(void* send_buffer,
             void* receive_buffer,
             int count,
             MPI_Datatype datatype,
             MPI_Op operation,
             MPI_Comm communicator);

Parameters

send_buffer

The buffer containing the value to send to scan. To use the in-place version, where the buffer in which store the result is also the buffer from which read the value to send, set the send_buffer to MPI_IN_PLACE.

receive_buffer

The variable in which store the result of the scan.

count

The number of elements in the scan.

datatype

The datatype of an element.

operation

The operation to apply to combine messages received in the scan. This operation must be associative, and commutative for predefined operations while user-defined operations may be non-commutative.

communicator

The communicator in which the scan takes places. MPI_Scan is not valid on inter-communicators.

Return value

The error code returned from the inclusive scan.

Example

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

/**
 * @brief Illustrate how to use an MPI_Scan.
 * @details This program uses MPI_Scan to compute a progressive sum of ranks. It
 * can be visualised as follows:
 *
 * +---------------+   +---------------+   +---------------+   +---------------+
 * | MPI process 0 |   | MPI process 1 |   | MPI process 2 |   | MPI process 3 |
 * +---------------+   +---------------+   +---------------+   +---------------+
 * |       0       |   |       1       |   |       2       |   |       3       |
 * +-------+-------+   +-------+-------+   +-------+-------+   +-------+-------+
 *         |                   |                   |                   |
 *         |                +--+--+                |                   |
 *         +----------------| SUM |                |                   |
 *         |                +--+--+                |                   |
 *         |                   |                +--+--+                |
 *         |                   +----------------| SUM |                |
 *         |                   |                +--+--+                |
 *         |                   |                   |                +--+--+
 *         |                   |                   +----------------| SUM |
 *         |                   |                   |                +--+--+
 *         |                   |                   |                   |
 * +-------+-------+   +-------+-------+   +-------+-------+   +-------+-------+
 * |       0       |   |       1       |   |       3       |   |       6       |
 * +---------------+   +---------------+   +---------------+   +---------------+
 * | MPI process 0 |   | MPI process 1 |   | MPI process 2 |   | MPI process 3 |
 * +---------------+   +---------------+   +---------------+   +---------------+
 *                                       
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

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

    // Get the sum of all ranks up to mine and print it
    int total;
    MPI_Scan(&my_rank, &total, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD);
    printf("[MPI process %d] Total = %d.\n", my_rank, total);

    MPI_Finalize();

    return EXIT_SUCCESS;
}