if (rank <= size / 2) {
MPI_Comm_create_group(MPI_COMM_WORLD, half_group, 0, &comm);
MPI_Barrier(comm);
MPI_Comm_free(&comm);
}
if (rank <= size / 2) {
MPI_Comm_create_group(MPI_COMM_WORLD, half_group, 0, &comm);
MPI_Barrier(comm);
MPI_Comm_free(&comm);
}