Process groups (MPI)

MPI_Comm_split partitions all processes by a color value, which works well when membership can be computed from a formula — even/odd rank, row and column in a grid. When the subset is defined by an explicit list of ranks with no clean formula, MPI_Comm_split has no natural expression for it. Process groups are the general mechanism: MPI_Group_incl builds a new group from an explicit list of ranks taken from an existing group, and MPI_Comm_create turns it into a communicator. MPI_Group_excl does the same by exclusion.

MPI_Group world_group, sub_group;
MPI_Comm sub_comm;
MPI_Comm_group(MPI_COMM_WORLD, &world_group);
 
int ranks[] = {0, 2, 4};
MPI_Group_incl(world_group, 3, ranks, &sub_group);
MPI_Comm_create(MPI_COMM_WORLD, sub_group, &sub_comm);
 
if (sub_comm != MPI_COMM_NULL) {
    // ranks 0, 2, 4 enter here with new ranks 0, 1, 2 in sub_comm
    do_work(sub_comm);
    MPI_Comm_free(&sub_comm);
}
MPI_Group_free(&sub_group);
MPI_Group_free(&world_group);

MPI_Comm_create is collective over the parent communicator: every process must call it, and those excluded receive MPI_COMM_NULL. MPI_Comm_create_group (MPI-3) is collective only over the group itself, which is more efficient when the subset is small. Groups are lightweight objects; they do not enable communication on their own and have no communication context until wrapped in a communicator.