[mpiwg-p2p] Message matching for tools

[Jeff Hammond]

How do you handle MPI_THREAD_MULTIPLE?  Understanding what your tool does
there is a good starting point for this discussion.

Jeff

On Wed, Dec 16, 2015 at 1:37 AM, Marc-Andre Hermanns <
hermanns at jara.rwth-aachen.de> wrote:

> Hi all,
>
> CC: Tools-WG, Markus Geimer (not on either list)
>
> sorry for starting a new thread and being so verbose, but I subscribed
> just now. I quoted Dan, Jeff, and Jim from the archive as appropriate.
>
> First, let me state that we do not want to prevent this assertion in
> any way. For us as tools provider it is just quite a brain tickler on
> how to support this in our tool and in general.
>
> Dan wrote:
> >>> [...] 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. [...]
> >>>
> >>> 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
> >
> Giving up can only be a temporary solution for tools. The user wants
> to use this advanced feature, thus just saying: "Hey, what you're
> doing is too sophisticated for us. You are on your own now." is not a
> viable long-term strategy.
>
> >>> 2) the tool could hook all send and receive operations and
> >>> piggy-back a sequence number into the message header
>
> We discussed piggy-backing within the tools group some time in the
> past, but never came to a satisfying way of how to implement this. If,
> in the process of reviving the discussion on a piggy-backing
> interface, we come to a viable solution, it would certainly help with
> the our issues with message matching in general.
>
> Scalasca's problem here is that we need to detect (and partly
> recreate) the exact order of message matching to have the correct
> message reach the right receivers.
>
> >>> 3) the tool could hook all send and receive operations and
> >>> serialise them to prevent overtaking
>
> This is not an option for me. A "performance tool" should strive to
> measure as close to the original behavior as possible. Changing
> communication semantics just to make a tool "work" would have too
> great of an impact on application behavior. After all, if it would
> have only little impact, why should the user choose this option in the
> first place.
>
> Jeff wrote:
> >> 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.
>
> This is true. If the tools can identify this scenario, it could be
> supported by current tools without significant change. However, as it
> is not generally forbidden to have inexact matching (right?), it is
> unclear on how the tools would detect this.
>
> What about an additional info key a user can set in this respect:
>
> exact_matching => true/false
>
> in which the user can state whether it is indeed a scenario of exact
> matching or not. The tool could check this, and issue a warning.
>
> >> If tools cannot handle MPI_THREAD_MULTIPLE already, then I don't really
> >> care if they can't support this assertion either.
>
> Not handling MPI_THREAD_MULTIPLE generally is not carved in stone. ;-)
>
> As I said, we (Markus and I) see this as a trigger to come to a viable
> solution for tools like ours to support either situation.
>
> >> And in any case, such tools can just intercept the info operations and
> >> strip this key if they can't support it.
>
> As I wrote above in reply to Dan, stripping options that influence
> behavior is not a good option. I, personally, would rather bail out
> than (silently) change messaging semantics. I can't say what Markus'
> take on this is.
>
> Jim wrote:
> > I don't really see any necessary fix to the proposal. We could add an
> > advice to users to remind them that they should ensure tools are
> compatible
> > with the info keys. And the reverse advice to tools writers that they
> > should check info keys for compatibility.
>
> I would second this idea, while emphasizing the burden to be on the
> tool to check for this info key (and potentially others) and warn the
> user of "undersupport".
>
> Cheers,
> Marc-Andre
> --
> Marc-Andre Hermanns
> Jülich Aachen Research Alliance,
> High Performance Computing (JARA-HPC)
> Jülich Supercomputing Centre (JSC)
>
> Schinkelstrasse 2
> 52062 Aachen
> Germany
>
> Phone: +49 2461 61 2509 | +49 241 80 24381
> Fax: +49 2461 80 6 99753
> www.jara.org/jara-hpc
> email: hermanns at jara.rwth-aachen.de
>
>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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