5.5.3 (Approximate) Current Practice #3

  main(int argc, char **argv)
  {
    int me, count, count2;
    void *send_buf, *recv_buf, *send_buf2, *recv_buf2;
    MPI_Group MPI_GROUP_WORLD, grprem;
    MPI_Comm commslave;
    static int ranks[] = {0};
    ...
    MPI_Init(&argc, &argv);
    MPI_Comm_group(MPI_COMM_WORLD, &MPI_GROUP_WORLD);
    MPI_Comm_rank(MPI_COMM_WORLD, &me);  /* local */

    MPI_Group_excl(MPI_GROUP_WORLD, 1, ranks, &grprem);  /* local */
    MPI_Comm_create(MPI_COMM_WORLD, grprem, &commslave);

    if(me != 0)
    {
      /* compute on slave */
      ...
      MPI_Reduce(send_buf,recv_buff,count, MPI_INT, MPI_SUM, 1, commslave);
      ...
    }
    /* zero falls through immediately to this reduce, others do later... */
    MPI_Reduce(send_buf2, recv_buff2, count2,
               MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);

    MPI_Comm_free(&commslave);
    MPI_Group_free(&MPI_GROUP_WORLD);
    MPI_Group_free(&grprem);
    MPI_Finalize();
  }
This example illustrates how a group consisting of all but the zeroth process of the ``all'' group is created, and then how a communicator is formed (commslave) for that new group. The new communicator is used in a collective call, and all processes execute a collective call in the MPI_COMM_WORLD context. This example illustrates how the two communicators (that inherently possess distinct contexts) protect communication. That is, communication in MPI_COMM_WORLD is insulated from communication in commslave, and vice versa.

In summary, ``group safety'' is achieved via communicators because distinct contexts within communicators are enforced to be unique on any process.

MPI-Standard for MARMOT