5.5.6 Library Example #2

The main program:
   main(int argc, char **argv)
   {
     int ma, mb;
     MPI_Group MPI_GROUP_WORLD, group_a, group_b;
     MPI_Comm comm_a, comm_b;

     static int list_a[] = {0, 1};
#if  defined(EXAMPLE_2B) | defined(EXAMPLE_2C)
     static int list_b[] = {0, 2 ,3};
#else/* EXAMPLE_2A */
     static int list_b[] = {0, 2};
#endif
     int size_list_a = sizeof(list_a)/sizeof(int);
     int size_list_b = sizeof(list_b)/sizeof(int);

     ...
     MPI_Init(&argc, &argv);
     MPI_Comm_group(MPI_COMM_WORLD, &MPI_GROUP_WORLD);

     MPI_Group_incl(MPI_GROUP_WORLD, size_list_a, list_a, &group_a);
     MPI_Group_incl(MPI_GROUP_WORLD, size_list_b, list_b, &group_b);

     MPI_Comm_create(MPI_COMM_WORLD, group_a, &comm_a);
     MPI_Comm_create(MPI_COMM_WORLD, group_b, &comm_b);

     if(comm_a != MPI_COMM_NULL)
        MPI_Comm_rank(comm_a, &ma);
     if(comm_a != MPI_COMM_NULL)
        MPI_Comm_rank(comm_b, &mb);

     if(comm_a != MPI_COMM_NULL)
        lib_call(comm_a);

     if(comm_b != MPI_COMM_NULL)
     {
       lib_call(comm_b);
       lib_call(comm_b);
     }

     if(comm_a != MPI_COMM_NULL)
       MPI_Comm_free(&comm_a);
     if(comm_b != MPI_COMM_NULL)
       MPI_Comm_free(&comm_b);
     MPI_Group_free(&group_a);
     MPI_Group_free(&group_b);
     MPI_Group_free(&MPI_GROUP_WORLD);
     MPI_Finalize();
   }

The library:

   void lib_call(MPI_Comm comm)
   {
     int me, done = 0;
     MPI_Comm_rank(comm, &me);
     if(me == 0)
        while(!done)
        {
           MPI_Recv(..., MPI_ANY_SOURCE, MPI_ANY_TAG, comm);
           ...
        }
     else
     {
       /* work */
       MPI_Send(..., 0, ARBITRARY_TAG, comm);
       ....
     }
#ifdef EXAMPLE_2C
     /* include (resp, exclude) for safety (resp, no safety): */
     MPI_Barrier(comm);
#endif
   }
The above example is really three examples, depending on whether or not one includes rank 3 in list_b, and whether or not a synchronize is included in lib_call. This example illustrates that, despite contexts, subsequent calls to lib_call with the same context need not be safe from one another (colloquially, ``back-masking''). Safety is realized if the MPI_Barrier is added. What this demonstrates is that libraries have to be written carefully, even with contexts. When rank 3 is excluded, then the synchronize is not needed to get safety from back masking.

Algorithms like ``reduce'' and ``allreduce'' have strong enough source selectivity properties so that they are inherently okay (no backmasking), provided that MPI provides basic guarantees. So are multiple calls to a typical tree-broadcast algorithm with the same root or different roots (see [28]). Here we rely on two guarantees of MPI : pairwise ordering of messages between processes in the same context, and source selectivity -- deleting either feature removes the guarantee that backmasking cannot be required.

Algorithms that try to do non-deterministic broadcasts or other calls that include wildcard operations will not generally have the good properties of the deterministic implementations of ``reduce,'' ``allreduce,'' and ``broadcast.'' Such algorithms would have to utilize the monotonically increasing tags (within a communicator scope) to keep things straight.

All of the foregoing is a supposition of ``collective calls'' implemented with point-to-point operations. MPI implementations may or may not implement collective calls using point-to-point operations. These algorithms are used to illustrate the issues of correctness and safety, independent of how MPI implements its collective calls. See also section 5.8.

MPI-Standard for MARMOT