Implement MPIX_GREQUEST_START

[ompiteam]

Rob Latham proposed MPIX_GREQUEST_START as described in "Extending the MPI-2 Generalized Request Interface":http://www-unix.mcs.anl.gov/~thakur/papers/grequest-redesign.pdf (PDF). Prototypes are in the paper (no use reproducing them here).

The main improvement is allowing generalized requests to specify their own progression function that will be invoked by MPI's progress engine. This MPIX_GREQUEST_START function is in MPICH2 and is now used by newer versions of ROMIO.

[ompiteam]

Imported from trac issue 1159. Created by jsquyres on 2007-10-07T05:43:12, last modified: 2008-06-21T14:06:51

[ompiteam]

Trac comment by jsquyres on 2008-06-21 14:06:51:

Not going to happen for v1.3.

[devreal]

The infrastructure for extended generalized requests has been around for quite a while in Open MPI (protected by --enable-grequest-extensions). Any opposition to adding the user-facing API for extended generalized requests? It's not in the standard but has been used in a paper at EuroMPI'19 and I've been hacking my way around using it with Open MPI myself.

[jsquyres]

As long as it's MPIX, I doubt there's a problem.

[jjhursey]

@ggouaillardet Do you know the state of this ticket? I think it can be closed. However, after looking at the code I think that some of the protections added for the --enable-grequest-extensions may have been removed by accident in this commit.

I see these two changes from you so I thought I would reach out to see what you think:

• Implement the extensions: ompi/requests: implement generalized request extensions #5337
• Make them internal: io/romio321: make grequest extensions internal #5353