Re: [Mpi3-ft] MPI_Comm_validate_all
Actually, I think Darius has a point. The exact guarantee in impossible in the general case because its reducible to the consensus problem. Unfortunately, the spec has to assume the general case, while databases don't need to and can assume synchronous communication or bounds on message delivery times. I think it'll be safer to use Darius' suggestion: guaranteed to return the same thing on processes where it does return something. Greg Bronevetsky Lawrence Livermore National Lab (925) 424-5756 [email protected] http://greg.bronevetsky.com
-----Original Message----- From: [email protected] [mailto:mpi3-ft- [email protected]] On Behalf Of Joshua Hursey Sent: Wednesday, February 16, 2011 1:17 PM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] MPI_Comm_validate_all
It is a challenging guarantee to provide, but possible. Databases need to make decisions like this all time with transactions (commit=success, or abort=failure). Though database transaction protocols are a good place to start, we can likely loosen some of the restrictions since we are applying them to a slightly different environment.
Look at a two-phase commit protocol that includes a termination protocol (Grey), or a three-phase commit protocol (Skeen). The trick is that you really want what the literature calls a 'nonblocking' commit protocol, meaning that it will not block in an undecided state waiting for the recovery of a peer process that might be able to decide from a recovery log. There are a few other more scalable approaches out there, but are challenging to implement correctly.
-- Josh
Gray: Notes on Data Base Operating Systems (note this describes a protocol without the termination protocol, but a databases text should be able to fill in that part) - 1979
Skeen: Nonblocking commit protocols - 1981
On Feb 16, 2011, at 3:49 PM, Darius Buntinas wrote:
MPI_Comm_validate_all, according to the proposal at [1], must "either
complete successfully everywhere or return some error everywhere." Is this possible to guarantee? What about process failures during the call? Consider the last message sent in the protocol. If the process sending that message dies just before sending it, the receiver will not know whether to return success or failure.
I think that the best we can do is say that the outcount and list of
collectively-detected dead processes will be the same at all processes where the call completed successfully.
Or is there a trick I'm missing?
Thanks, -d
web/wiki/ft/run_through_stabilization#CollectiveValidationOperations
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------------------------------------ Joshua Hursey Postdoctoral Research Associate Oak Ridge National Laboratory http://users.nccs.gov/~jjhursey
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Bronevetsky, Greg