build a collection of q communicators using MPI_Comm_split

/* comm_split.c -- build a collection of q communicators using MPI_Comm_split
*
* Input: none
* Output: Results of doing a broadcast across each of the q communicators.
*
* Note: Assumes the number of processes, p = q^2
*
* See Chap. 7, pp. 120 & ff in PPMPI
*/
#include
#include "mpi.h"
#include

main(int argc, char* argv[]) {
int p;
int my_rank;
MPI_Comm my_row_comm;
int my_row;
int q;
int test;
int my_rank_in_row;

MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &p);
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

q = (int) sqrt((double) p);

/* my_rank is rank in MPI_COMM_WORLD.
* q*q = p */
my_row = my_rank/q;
MPI_Comm_split(MPI_COMM_WORLD, my_row, my_rank,
&my_row_comm);

/* Test the new communicators */
MPI_Comm_rank(my_row_comm, &my_rank_in_row);
if (my_rank_in_row == 0)
test = my_row;
else
test = 0;

MPI_Bcast(&test, 1, MPI_INT, 0, my_row_comm);

printf("Process %d > my_row = %d, my_rank_in_row = %d, test = %d\n",
my_rank, my_row, my_rank_in_row, test);

MPI_Finalize();
} /* main */