On systems with receiver-pull rendezvous protocol, it is important for the
MPI_Isend's to arrive before the MPI_Irecv's are posted (more specifically,
before the code posts the last MPI call and enters a compute loop).
This example illustrates the effect of using the MPI_Isend calls first.
Note that this does not guarentee that the MPI_Isend's occur before the
MPI_Irecv's, only that they are more likely to. Other examples show how to
enforce the ordering.