2023 06 26 webex

we intensively discussed (thanks Rolf!) the different options for the changes in the MPI standard and I thought it's good to send this around as minutes.

Attendees (until the end): Joseph, Rolf, Dominik, Martin (Schreiber)

I’m trying to summarize it here:

First of all, there’s an alternative A and B:

• Alternative A: Changing MPI Sessions (starting/finishing) might lead to non-optimal utilization of MPI derived data types. This would be communicated to the implementors and they are supposed to fix.

** Obviously, this puts more effort for the implementors to get it done.

** We discussed the realization of this. There have been some thoughts that this would lead to overheads each time a communication is done to check for this, but then came to the conclusion that these overheads are always required: Data Types are defined without being specific to communicators. Hence, they need to be optimized in an lazy way or a- priory (or something in between). As a consequence, there would be always overheads and we don’t expect any additional overheads for this option A.

** This also follows the main principle of the MPI implementation to be easy usable.

• Alternative B: Derived data types might not be optimal anymore after changing MPI Sessions.

** This would put a lot of burden on the users.

** In particular, it’s not clear how to “recommit” datatypes in order to optimize this.

** Users don't want to live with the situation that data types might suddenly be less efficiently usable.

=> We (Rolf, Joseph, Dominik & Martin) all agreed to "Alternative A" (which doesn't mean that this is final)

The second issue is about "with or without option C":

• With option C:

** Requires users to care about everything on their own to free handlers at finalize.

** This breaks with libraries / tools to easily switch to sessions.

• Without option C:

** There would be just a recommendation to users to free handlers explicitly.

** This recommendation could be incentivize by tools such as MUST (pointing out the user that with Sessions, the handles, etc. should be freed).

=> We all agreed that without option C would be the best solution. Again, this is just our opinion and shouldn't be seen as a final decision (unless you all agree).

All the best,

Martin & the others

-- Prof. Dr. Martin Schreiber

Applied Mathematics | High Perf. Scientific Comp. for PDEs Université Grenoble Alpes | Lab. Jean Kuntzmann | INRIA, France

For Time-X Euro-HPC project: TUM School of CIT, Technical University of Munich, Germany