[Mpi3-ft] system-level C/R requirements

Sat Oct 25 13:03:21 CDT 2008

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
bronevetsky1 at llnl.gov

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: mpi3-ft-bounces at lists.mpi-forum.org
>[mailto:mpi3-ft-bounces at lists.mpi-forum.org] 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
>bronevetsky1 at llnl.gov
>
>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: mpi3-ft-bounces at lists.mpi-forum.org
> >[mailto:mpi3-ft-bounces at lists.mpi-forum.org] 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
> >bronevetsky1 at llnl.gov
> >
> >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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