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<p class="MsoNormal">Reply inline below –<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><b><span style="font-family:"-webkit-standard",serif;color:black">Question 1.</span></b><span style="font-family:"-webkit-standard",serif;color:black"> In our application there are two modules that run concurrently
and rely on MPI (using two different communicators for isolation purposes). During the development time we observed the following behavior: when one of the modules sends a large buffer (say +1GB), the other one encountered a kind of contention problem where
its messages were held back through the duration of the large data transfer, even though second module messages were really small. Is this a known behavior? Is there any kind of message fragmentation implemented in MPICH that would allow concurrent message
progression? If not, are there any plans to implement such a feature?<o:p></o:p></span></p>
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<p class="MsoNormal">Yes, this is a known behavior. By default, mpich will impose a global lock to ensure correctness. In the latest 3.4 release, we have an experimental “multi-vci” support that allows operations in different communicators to progress concurrently.
You can build that by `--enable-thread-cs=per-vci –with-ch4-max-vcis=8`. `8` is arbitrary max to balance initialization overhead, but you can bump that up to 64. You also need set environment variable `MPIR_CVAR_CH4_NUM_VCIS=8` to enable the multi-vci during
runtime. Again, 8 is arbitrary up to the configured maximum.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><b><span style="font-family:"-webkit-standard",serif;color:black">Question 2.</span></b><span style="font-family:"-webkit-standard",serif;color:black"> We already have used one of the scenarios described in the
previous email, where one thread of a process A sends a message to one of multiple threads on a process B, all of which are waiting on the same message triple through non-blocking receives. The expected behaviour would be that only one of the threads from
process B would receive the message, but, during our tests we have faced a failure where a segfault would be thrown from inside <i>MPI_Recv<span class="apple-converted-space"> </span></i>function at one of the receiving threads. Is the expected behavior correct?
Is there any known issue with this use case that would trigger the described problem? I know this can probably be the application's fault, I just want to check if there is any known issue on the matter.<o:p></o:p></span></p>
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<p class="MsoNormal">This sounds like a potential bug. Could you file an issue at
<a href="https://giithub.com/pmodels/mpich/issues">https://giithub.com/pmodels/mpich/issues</a>? We’ll need more details and ideally a reproducer to effectively debug.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">--<o:p></o:p></p>
<p class="MsoNormal">Hui Zhou<o:p></o:p></p>
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