[Mpi3-ft] simplified FT proposal
Anthony Skjellum
tony at runtimecomputing.com
Mon Jan 16 14:46:56 CST 2012
Satantan,
Here are my further thoughts, which hopefully includes an answer to your
questions :-)
1) Everything is set to "ERRORS return"; the goal is to get local errors
back if they are available.
2) I would emphasize non-blocking operations, but blocking operations
implemented with internal timeout could return a timeout-type error
3) You don't have to return the same return code, or the same results in
all processes in the communicator, you can get erroneous results
or local failures; the functions are also allowed to produce incorrect
results [and we should then discuss what error reporting means here...
I am happy with local errors returned where known, recognizing those
processes may die before the bottom of the block. However, I also
expect the implementation to do its best to propagate error state
knowledge within this FTBLOCK, based organically on ongoing communication
or on gossip if an implementation so chose.]
Also, because we assume that there is also algorithmic fault tolerance
at work, local errors may be raised by the application because it is doing
checks
for validity etc.
So, either MPI or the application may raise local errors prior to the
bottom of the FTBLOCK, and the success of the bottom of the block must be
allowed
to be fail, just based on ABFT inputs from the application to MPI, not
just based on MPI's opinion.
4) If you are willing to do everything nonblocking, then I can describe the
test at the bottom of the FTBLOCK as follows:
The test operation at the bottom of the FTBLOCK is effectively a
generalized WAIT_ALL, that completes or doesn't complete all the outstanding
requests, returning errors related to the faults observed, and providing a
unified 0/1 success failure state consistently across the group of comm
[or surviving members thereof].
In my view, the application as well as MPI can contribute error state as
input to the FTBLOCK test.
Also, the application that gets local errors inside the loop already is
immediately ready to punt at that point, and do a jump to the bottom of the
loop. Let's
assume it is required to do all the operations before getting to the bottom
of the loop, for now, and we just allow that some of these may return
further
errors (I am trying to keep it simple), and MPI-like rules of all
attempting the operations. If we can get this to work, we can weaken it
later.
There is no good way to describe some BLOCKING, some nonblocking
operations, because we have no descriptor to tell us if something failed
that previously
returned... and did not give a local error, so I am not going to pursue
BLOCKING for now. Let's assume we cannot do BLOCKING, and weaken this
later,
if we can get a consistent solution using all nonblocking.
Please tell me what you think.
Thanks for responding!
Tony
On Mon, Jan 16, 2012 at 12:39 PM, Sur, Sayantan <sayantan.sur at intel.com>wrote:
> Hi Tony,****
>
> ** **
>
> In the example semantics you mentioned, are the “ops” required to return
> the same result on all processors? Although this doesn’t change the API
> “op”, but it does change completion semantics of almost all MPI ops. I hope
> I am correctly interpreting your message.****
>
> ** **
>
> Thanks.****
>
> ** **
>
> ===****
>
> Sayantan Sur, Ph.D.****
>
> Intel Corp.****
>
> ** **
>
> *From:* mpi3-ft-bounces at lists.mpi-forum.org [mailto:
> mpi3-ft-bounces at lists.mpi-forum.org] *On Behalf Of *Anthony Skjellum
> *Sent:* Sunday, January 15, 2012 7:06 PM
>
> *To:* MPI 3.0 Fault Tolerance and Dynamic Process Control working Group
> *Subject:* Re: [Mpi3-ft] simplified FT proposal****
>
> ** **
>
> Everyone, I think we need to start from scratch.****
>
> ** **
>
> We should look for minimal fault-tolerant models that are achievable and
> useful. They may allow for a combination of faults (process and network),
> but in the end, as discussed in San Jose:****
>
> ** **
>
> FTBLOCK****
>
> --------------****
>
> Start_Block(comm)****
>
> ** **
>
> op [normal MPI operation on communicator specified by Start_Block, or
> subset thereof]****
>
> op****
>
> op****
>
> ** **
>
> Test_Block(comm)****
>
> ** **
>
> Which either succeeds, or fails, on the whole list of operations, followed
> by ways to reconstruct communicators and add back processes (easily),****
>
> provides for 3-level fault tolerant model****
>
> ** **
>
> a) Simply retry if the kind of error at the Test_Block is retryable****
>
> b) Simply reconstruct the communicator, use algorithmic fault tolerance to
> get lost data, and retry the block****
>
> c) drop back to 1 or more levels of checkpoint-restart.****
>
> ** **
>
> We can envision in this model an unrolling of work, in terms of a
> parameter N, if there is a lot of vector work, to allow granularity control
> ****
>
> as a function of fault environment.****
>
> ** **
>
> In some sense, a simpler model, that provides for efforts to detect****
>
> a) by MPI****
>
> b) allowing application monitor to assert failure asynchronously to this
> loop****
>
> ** **
>
> provides a more general ability to have coverage of faults, including but
> not limited to process faults and possible network faults.****
>
> ** **
>
> It changes the API very little.****
>
> ** **
>
> It motivates the use of buffers, not zero copy, to support the fact that
> you may to roll-back a series of operations, thereby revealing fault-free
> overhead directly.****
>
> ** **
>
> Start_Block and Test_Block are collective and synchronizing, such as
> barriers. Because we have uncertainty to within a message, multiple
> barriers (as mentioned by George Bosilca K to me in a sidebar at the
> meeting).****
>
> ** **
>
> We try to get this to work, COMPLETELY, and ratify this in MPI 3.x, if we
> can. Once we have this stable intermediate form, we explore more****
>
> options.****
>
> ** **
>
> I think it is important to recognize that the reconstruction step,
> including readding processes and making new communicators may mean smarter
> Join operations. It is clear we need to be able to treat failures during
> the recovery process, and use a second level loop, possibly bombing out***
> *
>
> to checkpoint, if we cannot make net progress on recovery because of
> unmodeled error issues.****
>
> ** **
>
> The testing part leverages all the learning so far, but needn't be
> restricted to modeled errors like process faults. There can be modeled and
> unmodeled faults. Based on what fault comes up, the user application then
> has to decide how hard a retry to do, whether just to add processes,****
>
> whether just to retry the loop, whether to go to a checkpoint, whether to
> restart the APP. MPI could give advice, based on its understanding of****
>
> the fault model, in terms of sufficient conditions for "working harder" vs
> "trying the easiest" for fault models it understands somewhat for a system.
> ****
>
> ** **
>
> Now, the comments here are a synopsis of part of the side bars and open
> discussion we had in San Jose, distilled a bit. I want to know why ****
>
> we can't start with this, succeed with this, implement and test it, and
> having succeeded, do more in a future 3.y, y > x release, given user
> experience.****
>
> ** **
>
> I am not speaking to the choice of "killing all communicators" as with
> FT-MPI, or "just remaking those you need to remake." I think we need to
> resolve. Honestly, groups own the fault property, not communicators, and
> all groups held by communicators where the fault happened should be
> rebuilt, not all communicators... Let's argue on that.****
>
> ** **
>
> So, my suggestion is REBOOT the proposal with something along lines above,
> unless you all see this is no better.****
>
> ** **
>
> With kind regards,****
>
> Tony****
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> On Sun, Jan 15, 2012 at 8:00 PM, Sur, Sayantan <sayantan.sur at intel.com>
> wrote:****
>
> Hi Bill,****
>
> ****
>
> I am in agreement with your suggestion to have a collective over a
> communicator that returns a new communicator containing ranks “alive some
> point during construction”. It provides cleaner semantics. The example was
> merely trying to utilize the new MPI_Comm_create_group API that the Forum
> is considering.****
>
> ****
>
> MPI_Comm_check provides a method to form global consensus in that all
> ranks in comm did call it. It does not imply anything about current status
> of comm, or even the status “just before” the call returns. During the
> interval before the next call to MPI_Comm_check, it is possible that many
> ranks fail. However, the app/lib using MPI knows the point where everyone
> was alive.****
>
> ****
>
> Thanks.****
>
> ****
>
> ===****
>
> Sayantan Sur, Ph.D.****
>
> Intel Corp.****
>
> ****
>
> *From:* mpi3-ft-bounces at lists.mpi-forum.org [mailto:
> mpi3-ft-bounces at lists.mpi-forum.org] *On Behalf Of *William Gropp
> *Sent:* Sunday, January 15, 2012 2:41 PM
> *To:* MPI 3.0 Fault Tolerance and Dynamic Process Control working Group
> *Subject:* Re: [Mpi3-ft] simplified FT proposal****
>
> ****
>
> One concern that I have with fault tolerant proposals has to do with races
> in the specification. This is an area where users often "just want it to
> work" but getting it right is tricky. In the example here, the
> "alive_group" is really only that at some moment shortly before
> "MPI_Comm_check" returns (and possibly not even that). After that, it is
> really the "group_of_processes_that_was_alive_at_some_point_in_the_past".
> Since there are sometimes correlations in failures, this could happen even
> if the initial failure is rare. An alternate form might be to have a
> routine, collective over a communicator, that returns a new communicator
> meeting some definition of "members were alive at some point during
> construction". It wouldn't guarantee you could use it, but it would have
> cleaner semantics.****
>
> ****
>
> Bill****
>
> ****
>
> On Jan 13, 2012, at 3:41 PM, Sur, Sayantan wrote:****
>
> ** **
>
> I would like to argue for a simplified version of the proposal that covers
> a large percentage of use-cases and resists adding new “features” for the
> full-range of ABFT techniques. It is good if we have a more pragmatic view
> and not sacrifice the entire FT proposal for the 1% fringe cases. Most apps
> just want to do something like this:****
>
> ****
>
> for(… really long time …) {****
>
> MPI_Comm_check(work_comm, &is_ok, &alive_group);****
>
> if(!is_ok) {****
>
> MPI_Comm_create_group(alive_group, …, &new_comm);****
>
> // re-balance workload and use new_comm in rest of computation****
>
> MPI_Comm_free(work_comm); // get rid of old comm****
>
> work_comm = new_comm;****
>
> } else {****
>
> // continue computation using work_comm****
>
> // if some proc failed in this iteration, roll back work done in this
> iteration, go back to loop****
>
> }****
>
> }****
>
> ****
>
> ****
>
> William Gropp****
>
> Director, Parallel Computing Institute****
>
> Deputy Director for Research****
>
> Institute for Advanced Computing Applications and Technologies****
>
> Paul and Cynthia Saylor Professor of Computer Science****
>
> University of Illinois Urbana-Champaign****
>
> ****
>
> ****
>
> ****
>
>
> _______________________________________________
> mpi3-ft mailing list
> mpi3-ft at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft****
>
>
>
> ****
>
> ** **
>
> --
> Tony Skjellum, PhD
> RunTime Computing Solutions, LLC
> tony at runtimecomputing.com
> direct: +1-205-314-3595
> cell: +1-205-807-4968****
>
> _______________________________________________
> mpi3-ft mailing list
> mpi3-ft at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft
>
--
Tony Skjellum, PhD
RunTime Computing Solutions, LLC
tony at runtimecomputing.com
direct: +1-205-314-3595
cell: +1-205-807-4968
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