## 5.5.6. Library Example #2

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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 Formalizing the Loosely Synchronous Model .

Up: Motivating Examples Next: Inter-Communication Previous: Library Example #1

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