if (my_rank < my_size-1)
        {
      /*send top most strip up one node to be recieved as bottom halo*/
      MPI_Isend(&phi[1][0], grid_size  , MPI_DOUBLE, down, 1, MPI_COMM_WORLD, &sreqU);
      /*recv top halo from up one node*/
      MPI_Irecv(phi[slice+1][0], grid_size, MPI_DOUBLE, down, 2, MPI_COMM_WORLD, &reqU);
    }

if (my_rank > 0)
    {
      /*recv top halo from down one node*/
      MPI_Irecv(&phi[0][0], grid_size , MPI_DOUBLE, up, 2, MPI_COMM_WORLD, &reqD);
      /*send bottom most strip down one node to be recieved as top halo*/
      MPI_Isend(&phi[slice][0], grid_size , MPI_DOUBLE, up, 1, MPI_COMM_WORLD, &sreqD);
    }

      printf("send/recieve complete on rank %dn", my_rank);