#include <stdio.h>
#include <math.h>
#include "mpi.h"
/* This example handles a 12 x 12 mesh, on 4 processors only. */
#define maxn 12
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];
double x[maxn][maxn];
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 */
MPI_Gather( xlocal[1], maxn * (maxn/size), MPI_DOUBLE,
x, maxn * (maxn/size), 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;
}