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<TITLE>Re: [Mpi3-ft] Summary of today's meeting</TITLE>
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<FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>Can someone think of a reason to have the library do this over the app ? I can see that letting the library do this will avoid potential race conditions that could arise if we let the app do this - basically out of band with respect to the expected communications traffic.<BR>
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Rich<BR>
<BR>
<BR>
On 10/21/08 11:52 PM, "Thomas Herault" <<a href="herault.thomas@gmail.com">herault.thomas@gmail.com</a>> wrote:<BR>
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Le 21 oct. 08 à 22:06, Howard Pritchard a écrit :<BR>
<BR>
> Hello Rich,<BR>
><BR>
> I thought it was also agreed that if process A communicates with <BR>
> failed process B<BR>
> which had been restarted by another process C, and this was the <BR>
> first communication<BR>
> from A to B since the restart of B, A would receive the equivalent <BR>
> of a ECONNRESET error.<BR>
> This was in the context of a case where option 5 below is not being <BR>
> used by the app.<BR>
><BR>
> Howard<BR>
><BR>
<BR>
Hello Howard,<BR>
<BR>
there was still some discussions about this at the end of the session.<BR>
<BR>
The argument is that the application could do as well as the library <BR>
to enforce this detection if this is needed: when a process is <BR>
launched to replace another one, it could define a new revision/epoch/<BR>
restart number and tag each communication with this number to <BR>
implement the check. If this can be done as efficiently by the <BR>
application as it would be done by the library, asking the application <BR>
to do it itself would help the library to avoid the additional cost <BR>
(i.e. piggybacking an integer to each message) when the application <BR>
does not need that functionality.<BR>
<BR>
It was suggested that the library could provide a generic mean to <BR>
piggyback this kind of information to each message, in a way similar <BR>
as what is discussed about piggyback/message logging-based fault <BR>
tolerance.<BR>
<BR>
Thomas<BR>
<BR>
> Richard Graham wrote:<BR>
>><BR>
>> Here is a summary of what I think that we agreed to today. Please <BR>
>> correct any errors, and add what I am missing.<BR>
>><BR>
>> • We need to be able to restore MPI_COMM_WORLD (and it’s <BR>
>> derivatives) to a usable state when a process fails.<BR>
>> • Restoration may involve having MPI_PROC_NULL replace the lost <BR>
>> process, or may replaced the lost processes with a new process <BR>
>> (have not specified how this would happen)<BR>
>> • Processes communicating directly with the failed processes will <BR>
>> be notified via a returned error code about the failure.<BR>
>> • When a process is notified of the failure, comm_repair() must be <BR>
>> called. Comm_repair() is not a collective call, and is what will <BR>
>> initiate the communicator repair associated with the failed process.<BR>
>> • If a process wants to be notified of process failure even if it <BR>
>> is not communicating directly with this process, it must register <BR>
>> for this notification.<BR>
>> • We don’t have enough information to know how to continue with <BR>
>> support for checkpoint/restart.<BR>
>> • We need to discuss what needs to do with respect to failure of <BR>
>> collective communications.<BR>
>><BR>
>> There are several issues that came up with respect to these, which <BR>
>> will be detailed later on.<BR>
>><BR>
>> Rich<BR>
>><BR>
>> _______________________________________________<BR>
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>><BR>
><BR>
><BR>
> --<BR>
><BR>
> Howard Pritchard<BR>
> Cray Inc.<BR>
><BR>
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