[mpiwg-rma] MPI_WIN_CREATE for intercommunicators

Thomas Jahns jahns at dkrz.de
Tue Jan 21 12:28:08 CST 2014


should this be the wrong mailing list[1], I apologize in advance and hope for
directions to the correct place to pose my question.

From the wording of the MPI 2.2 and 3.0 standards I can see (first
paragraph of 11.2.1 in MPI 2.2, procedurce specification of
MPI_WIN_CREATE in MPI 3.0) that MPI_WIN_CREATE is only allowed for

I was wondering why this restriction is there, when

1. cases where an intercommunicator is more clear for point-to-point
   communications, communications using rma would equally gain in
   clarity with an intercommunicator (from my POV at least),

2. the required resources (because the number of potential
   communication paths would be much lower) could be reduced because
   the intercommunicator not only specifies two groups communicating
   with each other but also not communicating (at least via the
   intercommunicator) within each group. I.e. when it is already known
   that the any two processes within one of the two groups of an
   intercommunicator would not issue RMA calls for the other,
   corresponding resources need not be reserved. Intra-communicator RMA
   on the other hand leaves the possibility of any communication pair

   Clearly with two groups of processes of sizes N and M the number of
   potential pairs is N * M where a corresponding intercommunicator is
   concerned but (N * M)**2 for an intracommunicator and

3. the semantics appropriate for an intercommunicator should be easy

Since allowing intercommunicators in MPI_WIN_CREATE would consequently

1. be aesthetically pleasing,
2. offer increased potential for efficient execution,
3. seem to have an easy proof-of-concept implementation strategy and
4. forms a pattern arising naturally from the rest of the standard in my eyes,

I'm clearly missing some ambiguity or technical/definition difficulty
here, but can't figure out which, why else would the standard
explicitly restrict MPI_WIN_CREATE to intracommunicators if the above
were the whole story?

Kind regards,
Thomas Jahns

[1] I've done some searching and came up with either
<mpiwg-rma at lists.mpi-forum.org> or <mpi-comment at lists.mpi-forum.org>
Thomas Jahns
DKRZ GmbH, Department: Application software

Deutsches Klimarechenzentrum
Bundesstraße 45a
D-20146 Hamburg

Phone: +49-40-460094-151
Fax: +49-40-460094-270
Email: Thomas Jahns <jahns at dkrz.de>

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