[Mpi3-ft] Summary of today's meeting

[Greg Bronevetsky]

There is one caveat here that we should be aware 
of. There is not efficient way to implement this 
if we want the sender of a message to be informed 
that the receiver has been reset because then 
every send becomes a send-receive, which will 
significantly reduce performance. However, if 
we're willing to wait until the process receives 
data from the reset process either directly or 
via some dependence through other processes, then 
all this can be implemented efficiently.

Also, we should keep in mind that for some 
protocols we need both piggybacking and 
non-blocking collectives. The latter is to avoid 
race conditions where a process has begun a 
blocking collective call but needs to be informed 
of something having to do with the communication.

Greg Bronevetsky
Post-Doctoral Researcher
1028 Building 451
Lawrence Livermore National Lab
(925) 424-5756
bronevetsky1 at llnl.gov

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