We're trying to come up with a good set of semantics that would serve a variety of system-level C/R vendors. I'm sure that if we target the needs of EverGrid or BLCR on Linux then we will succeed. However, that is not our goal. Our goal is to target user-level, kernel-level and VMM-level C/R (or hybrids) on the full range of platforms that may be supported by MPI. Focusing on Linux for the moment, lets look at the difference between user-level C/R and VMM-level C/R. VMM-level C/R sees a very large fraction of the system, meaning that it will likely be acceptable for the MPI_Prepare call to pull all message state into CPU or network card memory and just ensure that there is not message data on the wires and the switches. User-level C/R is much less capable, meaning that all MPI state must be not just in CPU RAM, but specifically in application-accessible memory space and may not have any problematic kernel-level attributes such as pinned/unpinned status. When interfacing a given C/R tool with MPI these details become important and we must specify them explicitly. If we do not, we'll devolve into the current situation where each C/R tool must interface with each MPI vendor to get things to work. Moving to other platforms that may not have the same definitions of user-level/kernel-level/VMM-level as does Linux, we have a much deeper problem. A given type of checkpointer virtualizes the system at a given level of abstraction, meaning that in order to work with such a checkpointer MPI_Prepare must move all MPI state above that level. Even if we take the most conservative approach of forcing MPI_Prepare to move MPI state to the highest level possible, this still has the basic language problem of specifying what this might mean on platforms that don't even exist today. I just don't know how to do that. As such, the only plausible way that I can see for us to provide checkpointing support within the MPI standard is to move this virtualization all the way to the level of the MPI specification and keep MPI internals as a black box. If the application wants to checkpoint MPI, it must use the MPI interface to checkpoint the application-visible state. If it wants to checkpoint MPI datatypes, it can use PMPI to wrap all datatype management calls and keep track of them on its own. If it wants to checkpoint message state, it can use piggybacking and the standard MPI communication calls to do the appropriate logging and coordination. We may want to add extra MPI calls to facilitate this (piggybacking is one such example) but the main point of this approach is that it works at a level that is natural for the MPI specification and doesn't force us to define internal details that have so far been left unspecified by the standard. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 [email protected] At 03:09 PM 10/24/2008, Supalov, Alexander wrote:

Thanks. I think the word "how" below is decisive.

The definition of MPI_Init and MPI_Finalize do not say "how" processes are created, and still, they work. Likewise, as soon as we can define the expected outcome of the proposed calls, we can offload the "how" to the system - in this case, the CR system.

Now we come to the expected outcome. Imagine we guarantee that there's no MPI communication between the PREPARE and RESTORE calls, and no messages stuck in the wire or in the buffers. What can be stored in the system memory covered by CR will be stored there. The rest will be restored by the RESTORE call once it gets control over this memory image back. This may include reinitialization of the networking hardware, reestablishment of connections, reopening of the files, etc.

What other guarantees do CR people want?

-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Friday, October 24, 2008 11:38 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] system-level C/R requirements

The imprecision comes from the MPI library's interactions with the C/R tool. It seems to me that each tool/MPI library combo will have to define their own semantics for the two calls. The only standard here would be that these functions would in fact need to be called. But at this point its not really an API but a mild constraint on how the real API would be used. As such, it doesn't carry much information about how checkpointing is to be done. What the system-level C/R people need is a set of guarantees about where MPI will put its state between the two calls. I don't see a way to give them such guarantees in a standardized way. This is where we get stuck: we either provide a couple of calls that has little informational content but rather server as placeholders for a real or go fully detailed on a platform-by-platform basic. Neither approach is likely to pass by the wider forum, which is why I don't know how to satisfy this need within the MPI 3.0 effort.

It looks to me like we're standardizing something that is already non-standard.

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

At 02:27 PM 10/24/2008, Supalov, Alexander wrote:

...

Thanks. Can (or should) one define semantics better than those of the MPI_INIT and MPI_FINALIZE? MPI job starts after MPI_Init. The job ends after MPI_Finalize. What happens before and after is almost undefined. This is about all the standard specifies, and it's rather clear why: it cannot prescribe the way in which processes are started, because it's very system specific. CR is possibly even more system specific.

Let's get back to the proposal:

MPI_PREPARE_FOR_CHECKPOINT(MPI_COMM) ~ MPI_FINALIZE MPI_RESTORE_AFTER_CHECKPOINT(MPI_COMM) ~ MPI_INIT

Use MPI_COMM_WORLD for global CR. Use MPI_COMM_SELF for local CR.

Call the first function immediately before the checkpoint, do the checkpoint the way you like, and call the second immediately after to re-enter the MPI session where you left it.

What else can be added to make this more clear and more precise than MPI_INIT and MPI_FINALIZE definitions?

-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Friday, October 24, 2008 11:14 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] system-level C/R requirements

I think that the problem for the forum will be the unclear semantics of the new calls. MPI_Init is not a good example because it has clear semantics for all users of MPI but not system-level services. The difference with the quiscence calls is that we're trying to provide a way to by-pass to regular MPI semantics and plug into the middle of MPI without precisely defining how the by-pass works. Precise semantics didn't matter for MPI_Init exactly because there has never been a way to look into the MPI implementation until now. The solution to this is to provide very loose semantics to the new calls but this just means that there will actually be no standard way to use the new calls, which is why I'm afraid the forum will not like it.

I can think of only two things that we can compare these calls to. The first is the proposed performance hint API. However, this API is just about hints and may not be a good enough analogy for the rest of the forum. The other analogy is the performance profiling APIs that some MPI implementation support. These APIs allow tools to determine some statistics about internal MPI state. If that is the analogy that is drawn, then it is bad for this proposal because I don't think that the performance profiling API ever got much support because of the issues that we're discussing here.

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

At 02:03 PM 10/24/2008, Supalov, Alexander wrote:

...

Thanks. I can't speak for the whole Forum, but my impression is that if the choice will be between solving the problem of MPI and CR on one hand, and not solving it on the other hand, a reasonable proposal will go a long way toward convincing the majority, or at least moving the discussion to a still better proposal.

As for the number of calls, this is question of ROI. We're going to add 200 or so fancy calls by the latest guess, while here we have just 2 that offer basic functionality of undeniable value. This should be acceptable.

...

Finally, I don't know a more implementation specific call than MPI_Init. The proposed calls live close nearby.

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