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Hi all,<br>
<div class="moz-forward-container"><br>
Markus has reminded me that we discussed the effect of the
mpi_assert_allow_overtaking assertion on tools (see below).<br>
<br>
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.<br>
<br>
My responses were:<br>
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<br>
2) the tool could hook all send and receive operations and
piggy-back a sequence number into the message header<br>
3) the tool could hook all send and receive operations and
serialise them to prevent overtaking<br>
I don't know the viability/acceptability of any of these
approaches.<br>
<br>
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.<br>
<br>
Cheers,<br>
Dan.<br>
<br>
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<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Subject:
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<td>MPI P2P "messages may overtake" assertion</td>
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<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Date: </th>
<td>Mon, 14 Dec 2015 16:24:44 +0100</td>
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<th align="RIGHT" nowrap="nowrap" valign="BASELINE">From: </th>
<td>Markus Geimer <a class="moz-txt-link-rfc2396E" href="mailto:m.geimer@fz-juelich.de"><m.geimer@fz-juelich.de></a></td>
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<th align="RIGHT" nowrap="nowrap" valign="BASELINE">To: </th>
<td><a class="moz-txt-link-abbreviated" href="mailto:dholmes@epcc.ed.ac.uk">dholmes@epcc.ed.ac.uk</a></td>
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<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">CC: </th>
<td>Marc-André Hermanns <a class="moz-txt-link-rfc2396E" href="mailto:m.a.hermanns@fz-juelich.de"><m.a.hermanns@fz-juelich.de></a></td>
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<br>
<pre>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
Fax: +49-2461-61-6656
E-Mail: <a class="moz-txt-link-abbreviated" href="mailto:m.geimer@fz-juelich.de">m.geimer@fz-juelich.de</a>
WWW: <a class="moz-txt-link-freetext" href="http://www.fz-juelich.de/jsc">http://www.fz-juelich.de/jsc</a>
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</pre>
<br>
<pre class="moz-signature" cols="72">--
Dan Holmes
Applications Consultant in HPC Research
EPCC, The University of Edinburgh
James Clerk Maxwell Building
The Kings Buildings
Peter Guthrie Tait Road
Edinburgh
EH9 3FD
T: +44(0)131 651 3465
E: <a class="moz-txt-link-abbreviated" href="mailto:dholmes@epcc.ed.ac.uk">dholmes@epcc.ed.ac.uk</a>
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