Thanks. We're talking about different time in the program life cycle. You're talking about an application that is already prepared to take advantage of FT support. I'm talking about the incentive to make a non-FT prepared application FT-ready. My expectation is that unless there's a guarantee that the work necessary to make a non-FT prepared application FT-ready will be supported by enough MPIs providing a reasonable level of FT support, people are unlikely to make that investment. And if this is true, then this will kill the FT in MPI-3 just as prolonged practical lack of uniform spawning support basically killed that MPI-2 feature, or at least made it practically irrelevant in the application sense. -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Friday, February 13, 2009 6:22 PM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward I'm not sure I understand your point. If an application is written to take advantage of the FT API, it will not need to be modified if it is moved from one MPI implementation to another. The only difference is that different subsets of physical errors will become recoverable/non-recoverable. The subsets in question will depend on the actual physical components in the system as well as the performance/reliability tradeoffs made by the MPI implementors to make the most out of the given system. A high-quality implementation will provide some knobs that will allow system administrators to tailor this tradeoff to their particular system. The FT API makes it possible for implementors to provide support for handling failures that is in any way that is richer than just aborting the applications. We cannot make constraints on how well they take advantage of this new dimension in user support. I guess the closest analogy to this is that MPI doesn't specify the latency and bandwidth of the network but does specify the semantics of MPI_Send and MPI_Recv. MPI implementations and system designers are free to satisfy these semantics in a way that optimizes performance, CPU overhead, scalability and system cost. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 [email protected] At 09:12 AM 2/13/2009, Supalov, Alexander wrote:

Hi,

Thanks. It appears to me that this goes a step farther than the spawning/attachment. That one was supposed to work everywhere, but very few implementation were available until very recently, and hence very few applications use this feature.

Now, with FT optional in the sense that you described, who's going to develop an application that may be able to take advantage of fairly unspecified capability to detect unknown number of faults? I can't imagine why this would happen.

Also, imagine one got used to a certain set of provided FT features. Going to another MPI implementation they may see another set of features, up to and including none at all. Why would they experiment? This locks users into one MPI implementation. No good.

So, I'd say that unless we can guarantee some basic functionality when FT is switched on that way or another, we're not going to create much excitement in the user community. And life's too short to work on something that nobody will be excited about.

It may be better to propose a way to control FT as a subset or what not, and then do guarantee that if you turn this capability on, you get as a minimum certain benefits by agreeing to certain tradeoff on performance.

That may be out of the scope of this particular WG, but this is something that this WG should very seriously consider in order for the outcome to fly. Or so I think.

Best regards.

Alexander

-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Friday, February 13, 2009 5:51 PM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward

...

FT must be optional to be accepted. One way is to put it into a subset, say, thru the MPI_INIT_ASSERTED proposal. Another is to add yet one MPI_INIT call that will contain a flag for FT configuration (like MPI_FT_SYNCHRONOUS, MPI_FT_ASYNCHRONOUS, etc.). This was mentioned in relation to the issue notification in the earlier fault handling=error handling proposal of mine, see https:// svn.mpi-forum.org/trac/mpi-forum-web/wiki/Fault%20Handling.

I agree that FT must be optional but I don't think that we need to add anything to the proposal to make this happen. The proposal provides an API that allows the MPI implementation to tell the application about detected but recoverable failures and help it perform recovery. It does not say anything about which failures must be recoverable for MPI. Reliable MPI implementations will be able to do much more than unreliable MPI implementations. Users who need reliability will choose the former while others will choose the latter. The same will apply for things like network degradation. Since the spec will never talk about what types of physical events must be reportable by MPI, individual implementations will be able to trade off efficiency against the usefulness of system monitoring and all such choices will be compliant to the spec.

Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 [email protected]

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