[mpiwg-hybridpm] First draft of Continuations section

[Jeff Hammond]

There are user-defined function signatures in Fortran for reductions.  Look
at 6.9.5 in https://www.mpi-forum.org/docs/mpi-4.0/mpi40-report.pdf.  Those
are callbacks that get invoked internally when reductions are called.

Jeff

On Wed, Aug 4, 2021 at 9:05 PM Joseph Schuchart via mpiwg-hybridpm <
mpiwg-hybridpm at lists.mpi-forum.org> wrote:

> Thanks Jeff! I am less worried about the out-of-band update of the
> status objects but the whole thing with callbacks and function pointers.
> The only place in the MPI standard using callbacks right now is the
> tools chapter and that is explicitly C-only. Looking at
> https://scicomp.stackexchange.com/a/286 it seems to be possible to pass
> around function pointers in Fortran but the comments suggest that its
> not compatible with C?
>
> Thanks
> Joseph
>
> On 8/4/21 1:00 PM, Jeff Hammond wrote:
> > Regarding Fortran…
> >
> > In theory, you need ASYNCHRONOUS/VOLATILE attributes on visible state
> that’s updated out of band.
> >
> > In practice, MPI implementations have done this with nonblocking
> send-recv for 25 years and it hasn’t been an issue.
> >
> > Sent from my iPhone
> >
> >> On Aug 4, 2021, at 7:36 PM, Joseph Schuchart via mpiwg-hybridpm <
> mpiwg-hybridpm at lists.mpi-forum.org> wrote:
> >>
> >> Dear all,
> >>
> >> I have written and attached a first draft of the continuations section
> that I would like to discuss in one of the upcoming meetings. Right now it
> is embedded at the end Section 3 (Point-to-Point following test and wait
> etc) where I felt it would fit well, I'm open to other suggestions though.
> >>
> >> There are still some open TODOs that I'd like to discuss. In
> particular, I'm grappling with the handling of statuses. As suggested
> during previous discussions, I removed the flag argument to MPI_Continue
> such that the MPI implementation is allowed to invoke the continuations
> directly (can be controlled through an info key though). That means that
> after the call to MPI_Continue there is no good reason to inspect the
> statuses anymore. However, we may still want the user to provide the status
> object(s) in order to avoid exposing implementation-internal memory in the
> callback. But then I am worried that this is a source of error, with users
> providing pointers to stack memory that goes out of scope before the
> continuation is invoked.
> >>
> >> Also, I am unsure whether and how this whole mechanism works with
> Fortran. The python generator happily generates the interface but I have no
> idea whether that is correct...
> >>
> >> Looking forward to discussing this on the call!
> >>
> >> Cheers
> >> Joseph
> >> <mpi40-report-continuations.pdf>
> >> _______________________________________________
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>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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