grid.c

/*
 * grid.c
 *
 *  build a 2d grid
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "lalib.h"


Lalib_grid * lalib_build_grid(MPI_Comm comm, int rows, int cols) {
  Lalib_grid *   grid;
  int            nproc;                    /* total number of procs in comm */
  int            my_rank;
  int            dims[LALIB_NDIMS];         /* array of dimensions */
  int            local[LALIB_NDIMS];        /*    local positions */
  int            period[LALIB_NDIMS];       /*    periodicities    */
  int            remain_dims[LALIB_NDIMS];  /* flags for row/col subgrids */
  
  /* check whether the processor count is correct */
  MPI_Comm_size(comm, &nproc);
  if (rows * cols != nproc) {
    fprintf(stderr, "wrong number of processes to build grid.\n");
    MPI_Abort(MPI_COMM_WORLD, -1);
    exit(-1);
  }

  grid = (Lalib_grid *) calloc(1, sizeof(Lalib_grid));
  grid->rows  = rows;
  grid->cols  = cols;

  /* generate new communicator with topology */
  dims[0] = rows;
  dims[1] = cols;
  period[0] = FALSE;
  period[1] = FALSE;
  MPI_Cart_create(comm, LALIB_NDIMS, dims, period, TRUE, &(grid->grid_comm));

  /* get local coordinates */
  MPI_Comm_rank(grid->grid_comm, &my_rank);
  MPI_Cart_coords(grid->grid_comm, my_rank, LALIB_NDIMS, local);
  grid->myrow = local[0];
  grid->mycol = local[1];

  /* split comm into row and column comm */
  remain_dims[0] = TRUE;
  remain_dims[1] = FALSE;
  MPI_Cart_sub(grid->grid_comm, remain_dims, &(grid->col_comm));
  
  remain_dims[0] = FALSE;
  remain_dims[1] = TRUE;
  MPI_Cart_sub(grid->grid_comm, remain_dims, &(grid->row_comm));
  
  return(grid);
}