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<p class="MsoNormal"><span style="font-size:11.0pt">HI All,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Following up on the Session’s related discussions we had at yesterday’s virtual forum.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">To review, we discussed three open issues:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><a href="https://github.com/mpi-forum/mpi-issues/issues/424">https://github.com/mpi-forum/mpi-issues/issues/424</a>
<br>
- Sessions: need to update sections of standard describing initial error handler<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><a href="https://github.com/mpi-forum/mpi-issues/issues/434">https://github.com/mpi-forum/mpi-issues/issues/434</a><br>
- Sessions: need to clarify limitations when mixing the sessions model with dynamic process model <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><a href="https://github.com/mpi-forum/mpi-issues/issues/435">https://github.com/mpi-forum/mpi-issues/issues/435</a><br>
- Is definition of local Session_init + collective Session_finalize pair broken?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">424 is easy to address and I’ll rebase PR 519 on top of the rc branch and we can maybe get it added to the agenda for next week.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">435 led to lots of lively conversation, but at the end of the day, my takeaway was that this is addressable via careful rewording and additional text plus some examples similar to the ones in 11.2.2 concerning
MPI_Finalize.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I have a problem though with some of the discussion around 434. I think it’s a big mistake to break with what had been our philosophy to date with sessions – that is don’t potentially break existing programs
and library stacks.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">The text near the beginning of chapter 11 currently has a blurb about interoperability:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="p1">An application can<o:p></o:p></p>
<p class="p1">employ both of these Process Models concurrently. In multi-component MPI applications,<o:p></o:p></p>
<p class="p1">for example, a component such as a library can make use of the Sessions Model to instantiate<o:p></o:p></p>
<p class="p1">MPI resources without impacting the rest of the application.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">And this<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="p1">Both of these models also support the Dynamic Process Model (see Section
<span class="s1">11.7</span>), which<o:p></o:p></p>
<p class="p1">provides for the creation and management of additional processes after an MPI application<o:p></o:p></p>
<p class="p1">has been started.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">If we were going to say that a communicator derived via the sessions model can’t be used with MPI_Comm_spawn or MPI_Comm_spawn_multiple then potentially an existing library which under the covers uses these
functions in its implementation, would stop working if the consumer of this library’s functionality (which probably involves some kind of input MPI communicator as an initialization argument) was converted to using sessions and supplied a communicator derived
via the sessions mechanism. I suspect if we opened a PR to declare that such communicators can’t be used with these spawn functions, then someone will want us to add a function to test whether a communicator was associated with a session. Then there would
be questions about MPI_Comm_accept/connect and so on.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">From an implementor’s point of view, I don’t see any major challenges supporting the dynamic process model with sessions – within the known limitations of having to use the world process model in the spawnee
processes.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I’m most reluctant to start putting in disclaimers and caveats about using communicators derived via the sessions mechanism as it seems to open us up to problems like these. Note I think some clarifying
text about limitations when using the dynamic process model in the context of sessions is definitely warranted, and will work on a PR to add such text to the chapter.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Howard<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">-- <o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;color:black">Howard Pritchard<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;color:black">HPC-ENV<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;color:black">Los Alamos National Laboratory<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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