Hello,
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
intracommunicators.
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
open.
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
to emulate with MPI_INTERCOMM_MERGE and MPI_GROUP_TRANSLATE_RANKS.
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(a)lists.mpi-forum.org> or <mpi-comment(a)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(a)dkrz.de>
