[mpiwg-sessions] MPI_Session_init semantics question/poll

Holmes, Daniel John daniel.john.holmes at intel.com
Wed Jan 4 13:05:14 CST 2023

Hi Howard,

It was always intended that MPI_Session_init was a local procedure. In fact, "initialise a session" is not even an MPI operation, so it doesn't make sense for it to be expressed via a nonlocal procedure.

Further, it was intended that the nonlocal portion of the work done by MPI_Init that is eventually needed in the pure sessions pattern would be done during the first nonlocal procedure call in that pattern, as follows:

MPI_Session_init // local - PMIx fence prohibited
MPI_Group_from_pset // local - PMIx fence prohibited
MPI_Comm_create_from_group // nonlocal - PMIx fence permitted, if needed

The nonlocal work should be unnecessary until the first nonlocal procedure call, so this should all work out fine (modulo some refactoring/debugging).

Best wishes,

From: mpiwg-sessions <mpiwg-sessions-bounces at lists.mpi-forum.org> On Behalf Of Pritchard Jr., Howard via mpiwg-sessions
Sent: 04 January 2023 18:32
To: MPI Sessions working group <mpiwg-sessions at lists.mpi-forum.org>
Cc: Pritchard Jr., Howard <howardp at lanl.gov>
Subject: [mpiwg-sessions] MPI_Session_init semantics question/poll

Hi All,

First, Happy New Year!

I've got a question about the semantics of MPI_Session_init.  In particular, I'd be interested in knowing  people's opinion on whether this function is nonlocal or local.
We don't have any text in the current version of the standard that states whether or not MPI_Session_init is a nonlocal operation.

I'm considering options for handling this issue:  https://github.com/open-mpi/ompi/issues/11166 .  It turns out that the way to properly resolve this issue depends on whether or not MPI_Session_init has local or nonlocal semantics.

I had been working under the assumption that we had intended session initialization to be a local function, but considering how to resolve issue 11166 made me begin to question this assumption.

Thanks for any ideas,



Howard Pritchard
Research Scientist

Los Alamos National Laboratory
howardp at lanl.gov<mailto:howardp at lanl.gov>


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