[mpiwg-p2p] Fwd: MPI P2P "messages may overtake" assertion

[Jeff Hammond]

Remember that one of the use cases of allow_overtaking is applications that
have exact matching, in which case allow_overtaking is a way of turning off
a feature that isn't used, in order to get a high-performing message queue
implementation.  In the exact matching case, tools will have no problem
matching up sends and recvs.

If tools cannot handle MPI_THREAD_MULTIPLE already, then I don't really
care if they can't support this assertion either.

And in any case, such tools can just intercept the info operations and
strip this key if they can't support it.

Jeff

On Mon, Dec 14, 2015 at 7:44 AM, Daniel Holmes <dholmes at epcc.ed.ac.uk>
wrote:

> Hi all,
>
> Markus has reminded me that we discussed the effect of the
> mpi_assert_allow_overtaking assertion on tools (see below).
>
> The basic problem is that message matching would be non-deterministic and
> it would be impossible for a tool to show the user which receive operation
> satisfied which send operation without internally using some sort of
> sequence number for each send/receive operation. As Markus mentions, this
> is a similar problem to the MPI_THREAD_MULTIPLE "logically concurrent"
> issue, with which tools already struggle.
>
> My responses were:
> 1) the user asked for this behaviour so the tool could simply gracefully
> give up the linking function and just state the information it knows
> 2) the tool could hook all send and receive operations and piggy-back a
> sequence number into the message header
> 3) the tool could hook all send and receive operations and serialise them
> to prevent overtaking
> I don't know the viability/acceptability of any of these approaches.
>
> Preferably before the formal reading at the next face-to-face meeting, we
> should discuss the possibility that this assertion will disrupt tools and
> ascertain if we can/want to do anything to mitigate that disruption.
>
> Cheers,
> Dan.
>
> -------- Forwarded Message -------- Subject: MPI P2P "messages may
> overtake" assertion Date: Mon, 14 Dec 2015 16:24:44 +0100 From: Markus
> Geimer <m.geimer at fz-juelich.de> <m.geimer at fz-juelich.de> To:
> dholmes at epcc.ed.ac.uk CC: Marc-André Hermanns <m.a.hermanns at fz-juelich.de>
> <m.a.hermanns at fz-juelich.de>
>
> Hi Daniel,
>
> Do you remember our conversation after the MPI BoF at SC'15 about the
> proposed "messages may overtake" assertion and that it may cause severe
> problems for tools, as correct message matching is very important for
> many of them?  In fact, the issue here is similar to properly supporting
> MPI_THREAD_MULTIPLE, which most tools don't do either right now...
>
> Meanwhile I talked to Marc-Andre Hermanns from the Tools WG -- which
> kick-started a discussion on this end.  But I guess this topic also
> requires some involvement of the P2P WG.  For example, it would be good
> if you could summarize the ideas you had in mind how tools could handle
> this assertion. Thanks!
>
> That said, I don't think there is any urgency for discussions, as
> people are slowly leaving for the Christmas break (e.g., I'm already
> out of office).  But we should definitely follow up on this early next
> year.
>
> Merry Christmas,
> Markus
>
> --
> Dr. Markus Geimer
> Juelich Supercomputing Centre
> Institute for Advanced Simulation
> Forschungszentrum Juelich GmbH
> 52425 Juelich, Germany
>
> Phone:  +49-2461-61-1773
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>
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> --
> Dan Holmes
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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