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 .