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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi Bill,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I am in agreement with your suggestion to have a collective over a communicator that returns a new communicator containing ranks “alive some point during construction”.
It provides cleaner semantics. The example was merely trying to utilize the new MPI_Comm_create_group API that the Forum is considering.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">MPI_Comm_check provides a method to form global consensus in that all ranks in comm did call it. It does not imply anything about current status of comm, or
even the status “just before” the call returns. During the interval before the next call to MPI_Comm_check, it is possible that many ranks fail. However, the app/lib using MPI knows the point where everyone was alive.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">===</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Sayantan Sur, Ph.D.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Intel Corp.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> mpi3-ft-bounces@lists.mpi-forum.org [mailto:mpi3-ft-bounces@lists.mpi-forum.org]
<b>On Behalf Of </b>William Gropp<br>
<b>Sent:</b> Sunday, January 15, 2012 2:41 PM<br>
<b>To:</b> MPI 3.0 Fault Tolerance and Dynamic Process Control working Group<br>
<b>Subject:</b> Re: [Mpi3-ft] simplified FT proposal<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">One concern that I have with fault tolerant proposals has to do with races in the specification. This is an area where users often "just want it to work" but getting it right is tricky. In the example here, the "alive_group" is really
only that at some moment shortly before "MPI_Comm_check" returns (and possibly not even that). After that, it is really the "group_of_processes_that_was_alive_at_some_point_in_the_past". Since there are sometimes correlations in failures, this could happen
even if the initial failure is rare. An alternate form might be to have a routine, collective over a communicator, that returns a new communicator meeting some definition of "members were alive at some point during construction". It wouldn't guarantee you
could use it, but it would have cleaner semantics.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Bill<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Jan 13, 2012, at 3:41 PM, Sur, Sayantan wrote:<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">I would like to argue for a simplified version of the proposal that covers a large percentage of use-cases and resists adding new “features” for the full-range of ABFT techniques.
It is good if we have a more pragmatic view and not sacrifice the entire FT proposal for the 1% fringe cases. Most apps just want to do something like this:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">for(… really long time …) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> MPI_Comm_check(work_comm, &is_ok, &alive_group);<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> if(!is_ok) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> MPI_Comm_create_group(alive_group, …, &new_comm);<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> // re-balance workload and use new_comm in rest of computation<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> MPI_Comm_free(work_comm); // get rid of old comm<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> work_comm = new_comm;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> } else {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> // continue computation using work_comm<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> // if some proc failed in this iteration, roll back work done in this iteration, go back to loop<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> }<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif";color:black">William Gropp<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif";color:black">Director, Parallel Computing Institute<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif";color:black">Deputy Director for Research<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif";color:black">Institute for Advanced Computing Applications and Technologies<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif";color:black">Paul and Cynthia Saylor Professor of Computer Science<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif";color:black">University of Illinois Urbana-Champaign<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:13.5pt;font-family:"Helvetica","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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