#include <stdio.h> #include <stdlib.h> #include <math.h> #include "mpi.h" /* This example handles a 12 x 12 mesh, on 4 processors only. */ #define maxn 12 /* This example handles precisely 4 processors */ #define maxp 4 int main( argc, argv ) int argc; char **argv; { int rank, value, size, errcnt, toterr, i, j, itcnt; int i_first, i_last; MPI_Status status; double diffnorm, gdiffnorm; double xlocal[(12/4)+2][12]; double xnew[(12/3)+2][12]; /* For the gatherv */ double x[maxn][maxn]; int lcnt; int recvcnts[maxp]; int displs[maxp]; MPI_Init( &argc, &argv ); MPI_Comm_rank( MPI_COMM_WORLD, &rank ); MPI_Comm_size( MPI_COMM_WORLD, &size ); if (size != 4) MPI_Abort( MPI_COMM_WORLD, 1 ); /* xlocal[][0] is lower ghostpoints, xlocal[][maxn+2] is upper */ /* Note that top and bottom processes have one less row of interior points */ i_first = 1; i_last = maxn/size; if (rank == 0) i_first++; if (rank == size - 1) i_last--; /* Fill the data as specified */ for (i=1; i<=maxn/size; i++) for (j=0; j<maxn; j++) xlocal[i][j] = rank; for (j=0; j<maxn; j++) { xlocal[i_first-1][j] = -1; xlocal[i_last+1][j] = -1; } itcnt = 0; do { /* Send up unless I'm at the top, then receive from below */ /* Note the use of xlocal[i] for &xlocal[i][0] */ if (rank < size - 1) MPI_Send( xlocal[maxn/size], maxn, MPI_DOUBLE, rank + 1, 0, MPI_COMM_WORLD ); if (rank > 0) MPI_Recv( xlocal[0], maxn, MPI_DOUBLE, rank - 1, 0, MPI_COMM_WORLD, &status ); /* Send down unless I'm at the bottom */ if (rank > 0) MPI_Send( xlocal[1], maxn, MPI_DOUBLE, rank - 1, 1, MPI_COMM_WORLD ); if (rank < size - 1) MPI_Recv( xlocal[maxn/size+1], maxn, MPI_DOUBLE, rank + 1, 1, MPI_COMM_WORLD, &status ); /* Compute new values (but not on boundary) */ itcnt ++; diffnorm = 0.0; for (i=i_first; i<=i_last; i++) for (j=1; j<maxn-1; j++) { xnew[i][j] = (xlocal[i][j+1] + xlocal[i][j-1] + xlocal[i+1][j] + xlocal[i-1][j]) / 4.0; diffnorm += (xnew[i][j] - xlocal[i][j]) * (xnew[i][j] - xlocal[i][j]); } /* Only transfer the interior points */ for (i=i_first; i<=i_last; i++) for (j=1; j<maxn-1; j++) xlocal[i][j] = xnew[i][j]; MPI_Allreduce( &diffnorm, &gdiffnorm, 1, MPI_DOUBLE, MPI_SUM, MPI_COMM_WORLD ); gdiffnorm = sqrt( gdiffnorm ); if (rank == 0) printf( "At iteration %d, diff is %e\n", itcnt, gdiffnorm ); } while (gdiffnorm > 1.0e-2 && itcnt < 100); /* Collect the data into x and print it */ /* This code (and the declaration of x above) are the only changes to the Jacobi code */ /* Use gather to get the recv counts */ lcnt = maxn * (maxn / size); MPI_Gather( &lcnt, 1, MPI_INT, recvcnts, 1, MPI_INT, 0, MPI_COMM_WORLD ); /* Form the displacements */ displs[0] = 0; for (i=1; i<size; i++) displs[i] = displs[i-1] + recvcnts[i-1]; MPI_Gatherv( xlocal[1], lcnt, MPI_DOUBLE, x, recvcnts, displs, MPI_DOUBLE, 0, MPI_COMM_WORLD ); if (rank == 0) { printf( "Final solution is\n" ); for (i=maxn-1; i>=0; i--) { for (j=0; j<maxn; j++) printf( "%f ", x[i][j] ); printf( "\n" ); } } MPI_Finalize( ); return 0; }