[mpich-discuss] Progress for user-defined non-blocking collectives

[Jed Brown]

Dave, it looks like you implemented the non-blocking collectives for MPI-3.
How reasonable do you think it would be to expose enough hooks to be able
to write a user-defined non-blocking collective that could make useful
progress?

This comes up pretty frequently where there is a high-level operation with
collective semantics that internally needs to perform multiple dependent
communications. We might still expose a non-blocking interface to the user,
but the performance benefit is limited because the multiple rounds either
take place up-front or at the end.

Generalized requests aren't (currently) a good solution because (from what
I can tell), they only make progress when _that request_ is polled. In
practice, you want to poke those requests from other library calls (or,
eventually/on some systems, by a progress thread), just like MPI native
operations can make progress without explicitly being polled.
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