POISSON_I

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


void
exchange(dmatrix a, Decomposition oned, Grid grid)
{
  /* exchange of boundary values (ghost points) using non-blocking routines */

   int         lx, ly;
   MPI_Status  status[4];
   MPI_Request req[4];

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

   /* extra boundary points in y are not exchanged, they are constant anyway */
   MPI_Irecv(&INDEX(a, lx+1, 1), ly, MPI_DOUBLE, grid.up, 0, 
            grid.comm, &req[0]);
   MPI_Irecv(&INDEX(a, 0, 1), ly, MPI_DOUBLE, grid.down,  1,
            grid.comm, &req[1]);
   MPI_Isend(&INDEX(a, 1, 1), ly, MPI_DOUBLE, grid.down, 0, 
             grid.comm, &req[2]);
   MPI_Isend(&INDEX(a, lx, 1), ly, MPI_DOUBLE, grid.up,  1,
             grid.comm, &req[3]);

   MPI_Waitall(4, req, status);
}