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<p class=MsoNormal>Hi mpich2-discuss,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>We’re in the process of moving to MPICH2-1.2.1p1 using the
SMPD/nemesis variant (on Linux only - on Windows, we’re waiting for a fix to
ticket #895), and we’ve found a discrepancy in the behaviour compared to the
sock variant. I’m not sure if this is a real bug, or if I’ve missed something
in the MPI standard. Our test for our wrapper around MPI_Barrier() essentially
proceeds as follows (see attached for a C test case which usually shows this
problem when running with 10 processes). Each process does this:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l1 level1 lfo2'><![if !supportLists]><span
style='mso-list:Ignore'>1.<span style='font:7.0pt "Times New Roman"'>
</span></span><![endif]>Call MPI_Send() to each other process in turn with a
tiny payload (assuming that this will be sent in the “eager” mode).<o:p></o:p></p>
<p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l1 level1 lfo2'><![if !supportLists]><span
style='mso-list:Ignore'>2.<span style='font:7.0pt "Times New Roman"'>
</span></span><![endif]>MPI_Barrier()<o:p></o:p></p>
<p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l1 level1 lfo2'><![if !supportLists]><span
style='mso-list:Ignore'>3.<span style='font:7.0pt "Times New Roman"'>
</span></span><![endif]>Check that MPI_Iprobe() indicates a message ready to
receive from each other process<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>With the sock variant, this works as I expect - each process
gets a return from MPI_Iprobe indicating that there is indeed a message waiting
from each other process. With nemesis, this isn’t always the case - sometimes
multiple calls to MPI_Iprobe are required. (Could this be related to ticket
#1062?).<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I couldn’t see in the MPI standard where the “expected”
behaviour of the above might be specified, but it’s possible that I’ve missed
something.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I can see several options for where a problem might exist:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo4'><![if !supportLists]><span
style='mso-list:Ignore'>1.<span style='font:7.0pt "Times New Roman"'>
</span></span><![endif]>MPI doesn’t actually specify that these MPI_Iprobe()s
should definitely return “true”<o:p></o:p></p>
<p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo4'><![if !supportLists]><span
style='mso-list:Ignore'>2.<span style='font:7.0pt "Times New Roman"'>
</span></span><![endif]>The nemesis channel isn’t preserving the ordering
between MPI_Barrier() and pt2pt communications in the way I expect<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>As it happens, our usage of MPI_Iprobe() is basically
restricted to our test code, so we could modify our tests not to rely on the
old behaviour, but we’d like to understand better where the problem is.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Cheers,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Edric.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
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