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" <[email protected]> wrote:

Le 21 oct. 08 à 22:06, Howard Pritchard a écrit :

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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

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Richard Graham wrote:

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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 it¹s 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 don¹t 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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