[Mpi3-rma] mpi3-rma post from bradc at cray.com requires approval

[Underwood, Keith D]

> > We would need to think about whether we have to have the whole
> > message ordered or ordered on a per target address basis.
> 
> Atomicity and ordering go hand-in-hand; if there's no atomicity,
> ordering doesn't make sense. Since we have basic datatype atomicity for
> accumulate/get_accumulate, ordering would make sense at that
> granularity
> as well.
> 
> If someone wants to propose full-message atomicity, then we can
> consider
> ordering at that granularity too. But till then, whole message ordering
> is an overkill.

Well, they aren't orthogonal, but they aren't quite that tightly linked.  A user that knew that two messages were not going to overlap might want to use a full message ordering from a single node for completion detection.  E.g. an MPI_Accumulate() with "replace" to one buffer and then an MPI_Accumulate() to another buffer to increment a variable and use the full message ordering to be able to use the latter for completion without the expense of a flush() between the messages.  So, it has value and a usage scenario.  I just don't know if we want to go that far or not.  

Keith