exchange.c

#include "array.h"
#include "poisson.h"


void
exchange(dmatrix a, Decomposition oned, Grid grid)
{
  /* exchanges the boundary values (ghost points) */

   int         lx, ly;
   MPI_Status  status;

   lx  =  oned.lx;
   ly  =  oned.ly;

   /* extra boundary points in y are not exchanged, they are constant anyway */
   MPI_Send(&INDEX(a, 1, 1), ly, MPI_DOUBLE, grid.down, TAG_DOWN, grid.comm);
   MPI_Recv(&INDEX(a, lx+1, 1), ly, MPI_DOUBLE, grid.up, TAG_DOWN, 
            grid.comm, &status);

   MPI_Send(&INDEX(a, lx, 1), ly, MPI_DOUBLE, grid.up,  TAG_UP, grid.comm);
   MPI_Recv(&INDEX(a, 0, 1), ly, MPI_DOUBLE, grid.down,  TAG_UP,
            grid.comm, &status);
}