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121 lines
3.7 KiB
121 lines
3.7 KiB
/*******************************************************************
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*
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* A similar example that uses MPI_Scatterv and MPI_Gatherv
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*
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* The program should be run with exactly 8 processes.
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* Process 0 initializes a matrix (x) of 8 by 8 integers and distributes
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* the lower triangular part of the matrix to the processes using
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* MPI_Scatterv. The processes receive different number of elements:
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* process 0 gets one element, process 1 gets 2 elements and process i
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* gets i+1 elements. The elements of all processes are then gathered
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* (using MPI_Gatherv) again in another 8 by 8 matrix (res) of process 0
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* (which is initialized with zeros) in triangular shape.
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*
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* ********************************************************************/
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#include <stdio.h>
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#include <stdlib.h>
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#include "mpi.h"
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#define MAXPROC 8 /* Max number of procsses */
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#define LENGTH 8 /* Size of matrix is LENGTH * LENGTH */
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main(int argc, char* argv[]) {
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int i, j, np, me;
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const int root = 0; /* Root process in scatter */
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int x[LENGTH][LENGTH]; /* Send buffer */
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int y[LENGTH]; /* Receive buffer */
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int res[LENGTH][LENGTH]; /* Final receive buffer */
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int *sendcount, *recvcount; /* Arrays for sendcounts and recvcounts */
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int *displs1, *displs2; /* Arrays for displacements */
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MPI_Init(&argc, &argv); /* Initialize MPI */
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MPI_Comm_size(MPI_COMM_WORLD, &np); /* Get nr of processes */
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MPI_Comm_rank(MPI_COMM_WORLD, &me); /* Get own identifier */
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/* Check that we have one process for each row in the matrix */
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if (np != LENGTH) {
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if (me == 0) {
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printf("You have to use %d processes\n", LENGTH);
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}
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MPI_Finalize();
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exit(0);
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}
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/* Allocate memory for the sendcount and displacements arrays */
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sendcount = (int *)malloc(LENGTH*sizeof(int));
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displs1 = (int *)malloc(LENGTH*sizeof(int));
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/* Initialize sendcount and displacements arrays */
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for (i=0; i<LENGTH; i++) {
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sendcount[i] = i+1;
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displs1[i] = i*LENGTH;
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}
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if (me == 0) { /* Process 0 does this */
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/* Initialize the matrix x with values 0 .. LENGTH*LENGTH-1 */
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for (i=0; i<LENGTH; i++) {
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for (j=0; j<LENGTH; j++) {
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x[i][j] = i*LENGTH+j;
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}
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}
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/* Print out the matrix before it is distributed */
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printf("The initial matrix is\n");
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for (i=0; i<LENGTH; i++) {
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for (j=0; j<LENGTH; j++) {
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printf("%4d ", x[i][j]);
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}
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printf("\n");
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}
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}
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/* Scatter the lower triangular part of array x to all proceses, place it in y */
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MPI_Scatterv(&x, sendcount, displs1, MPI_INT, &y, sendcount[me], MPI_INT, root, MPI_COMM_WORLD);
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if (me==0) {
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/* Initialize the result matrix res with values 0 */
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for (i=0; i<LENGTH; i++) {
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for (j=0; j<LENGTH; j++) {
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res[i][j] = 0;
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}
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}
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/* Print out the result matrix res before gathering */
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printf("The result matrix before gathering is\n");
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for (i=0; i<LENGTH; i++) {
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for (j=0; j<LENGTH; j++) {
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printf("%4d ", res[i][j]);
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}
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printf("\n");
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}
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}
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/* Allocate memory for the recvcount and displacements arrays */
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recvcount = (int *)malloc(LENGTH*sizeof(int));
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displs2 = (int *)malloc(LENGTH*sizeof(int));
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for (i=0; i<LENGTH; i++) {
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recvcount[i] = i+1;
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displs2[i] = i*LENGTH;
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}
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/* Gather the local elements of each process to form a triangular matrix in the root */
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MPI_Gatherv(&y, recvcount[me], MPI_INT, &res, recvcount, displs2, MPI_INT, root, MPI_COMM_WORLD);
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if (me == 0) { /* Process 0 does this */
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/* Print out the result matrix after gathering */
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printf("The result matrix after gathering is\n");
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for (i=0; i<LENGTH; i++) {
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for (j=0; j<LENGTH; j++) {
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printf("%4d ", res[i][j]);
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}
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printf("\n");
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}
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}
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MPI_Finalize();
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}
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