Consider the broadcast operation defined on page 124. The processes in group B
are all receiving data from the root (in group A). This is locally known since
the communicator is an inter-communicator, and the special (MPI_ROOT) value is
NOT passed as the root. The root process in group A is sending data to all the
processes in group B. This is locally known since the communicator is an
inter-communicator, and the special (MPI_ROOT) value IS passed as the root.
How do the other processes in group A know that they should not participate?
What should their root value be?
I believe the original intent here is that ONLY the root process in group A
calls the MPI_Bcast. Thus we redefine the semantics of collective (must be
called by all processes in group B, and ONLY the root in group A.)
This ugliness must either be repaired or detailed in the document.
Lloyd Lewins
Hughes Aircraft Co.,
llewins@msmail4.hac.com