Re: [mpiwg-ft] [EXTERNAL] Re: FTWG Con Call Today

21 Dec 2016

      That is a great idea, Keita, and I will be happy to participate to explore the relative areas of strength of these respective models. I think the first steps should be:

·        define a (small) collection of workloads that represent major MPI usage models, such as: master worker, SPMD, small number of communicators with mostly point-to-point communications, many different communicators with mostly collective operations, strictly iterative (identical iterations) versus semi-iterative (same iterations, but data set sizes change due to Adaptive Mesh Refinement, for example), versus non-iterative (some graph problems, such as betweenness centrality), multi-physics models, hybrid MPI/threading models, one-sided communications, applications that use external libraries. Of course, these models are not mutually exclusive. We shouldn’t go crazy here, but make sure we have a sufficient basis for evaluation, and then parameterize these workloads.

·        Define various fault modes to be covered.

Rob

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Teranishi, Keita
Sent: Tuesday, December 20, 2016 11:27 PM
To: [email protected]; MPI WG Fault Tolerance and Dynamic Process Control working Group <[email protected]>; Bland, Wesley <[email protected]>
Subject: Re: [mpiwg-ft] [EXTERNAL] Re: FTWG Con Call Today

Ignacio,

Yes, ReInit and Fenix-1.0 have the same recovery model. They use longjump for global rollback and fix MPI communicator at the end of "Init² call.  I am very happy to perform the feasibility studies of these three (plus one) models.  I think that it will be great if we can explore the feasibility through some empirical (prototyping) studies.

As for 4th (ReInit/Fenix-1.0) model, we should have clear definition MPI communicator recovery including subcommunicators.  In order to utilize the next generation checkpoint library (ECP¹s multi-level checkpointing

project) or accommodate application specific recovery schemes, MPI_Comm should provide some information of its past (history failures or change in the rank, comm_size, etc.) as well as its current state.  I am hoping that our experience with Fenix will help to design a new spec.

Thanks,

---------------------------------------------------------------------------

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Keita Teranishi

Principal Member of Technical Staff

Scalable Modeling and Analysis Systems

Sandia National Laboratories

Livermore, CA 94551

+1 (925) 294-3738

On 12/20/16, 4:55 PM, "Ignacio Laguna" <[email protected]<mailto:[email protected]>> wrote:
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Hi Keita,
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I think we all agree that there is no silver bullet solution for the FT
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problem and that each recovery model (whether it's ULFM, Reinit, Fenix,
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or ULFM+autorecovery) works for some codes but doesn't work for others,
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and that one of the solutions to cover all applications is to allow
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multiple recovery models.
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In the last telecon we discussed two ways to do that: (a) all models
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are compatible with each other; (b) they are not compatible, thus the
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application has to select the model to be used (which implies libraries
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used by the application have to support that model as well). The ideal
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case is (a), but we are not sure if it's possible, thus we are going to
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discuss each model in detail to explore that possibility. I believe
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case
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(b) is always a possibility, in which case you can still run Fenix on
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top of ULFM in that situation.
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BTW, correct me if I'm wrong, but Reinit and Fenix share (at a
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high-level) the same idea of global backward recovery with longjumps to
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reinject execution; thus we should call the 4rth option perhaps
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Reinit/Fenix.
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Ignacio
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On 12/20/16 3:06 PM, Teranishi, Keita wrote:
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All,
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Throughout the discussion, I am a bit worried about making MPI bigger
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than message passing interface because I wish MPI to serve a good
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abstraction of user-friendly transport layer.  Fenix is intended to
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leverage the minimalist approach of MPI-FT (ULFM today) to cover most
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of online recovery models for parallel programs using MPI.  The
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current version is designed to support SPMD (Communicating Sequential
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Process) model, but we wish to support other models including
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Master-Worker, Distributed Asynchronous Many Task (AMT) and Message-Logging.
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·ULFM: We have requested non-blocking communicator recovery as well as
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non-blocking comm_dup and comm_split, etc.   ULFM already provides good
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mechanism to serve master-worker type recovery like UQ, model
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reduction and a certain family of eigenvalue solvers.  I wish to have
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more fine control for revocation because it is possible to keep the
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certain connection of survived process (for master-worker or
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task-parallel computing), but it might be too difficult.
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·ULFM + Auto recovery: I need clarification from Wesly (as my
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knowledge is wrong most likelyŠ but let me continue based on my assumption).
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Fenix assumes that failure happens at a single or a small number of
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processes.  In this model, auto-recovery could serve as
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un-coordinated recovery because no comm_shrink call is used to fix the communicator.
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This could help message reply of uncoordinated recovery model.  For
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example, recovery is never manifested as ³Failure² to the survived
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ranks, making particular message passing calls very slow.   For SPMD
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model, adaptation is so challenging as the user needs to write how to
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recover the lost state of failed processes.  However, I can see a
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great benefit for implementing resilient task parallel programming model.
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·Communicator with hole: Master-Worker type applications will benefit
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from this when making collectives to gather the data available.
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·MPI_ReInit:  MPI_ReInit is very close to the current Fenix model.
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We have written the API specification (see attached) to support the
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same type of online recovery (global rollback upon process failure).
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The code is implemented using MPI-ULFM, and we have seen some issues
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with MPI-ULFM that makes multiple communicator recovery convoluted.
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We used PMPI to hide all the details of error handling, garbage
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collection and communicator recovery. The rollback (to Fenix_Init) is
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performed through longjmp.  Nice features of Fenix are (1) an idea of
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*resilient
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communicator* that allows the users to specify which communicator
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needs to be automatically fixed and (2) *callback functions* to
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assist application-specific recovery followed by communicator
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recovery.  We originally do not intend Fenix to be part of the MPI
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standard because we want the role of MPI confined within ³Message Passing² and do not want
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delay the MPI standardization discussions.    My understanding with
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MPI_ReInit is standardizing online-rollback recovery and keeping
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PMPI/QMPI layer clean through a tight binding with the layers
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invisible to typical MPI users (or tool developers) --- Ignacio,
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please correct me if I am wrong.  My biggest concern of MPI_ReInit is
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that defining rollback model by Message Passing Library may violate
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the original design philosophy of MPI (again this is the reason why
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we did not propose Fenix as MPI standard).  Another concern is that
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it might be difficult to keep other recovery options open, but it
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gets much more flexible with a few knobs in the APIs.  I think the
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latter is easy to fix with some switches in APIs.  I think we can
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figure out the options as we discuss further.
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Thanks,
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Keita
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*From: *"Bland, Wesley" <[email protected]<mailto:[email protected]>>
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*Date: *Tuesday, December 20, 2016 at 1:48 PM
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*To: *MPI WG Fault Tolerance and Dynamic Process Control working
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Group <[email protected]<mailto:[email protected]>>, "Teranishi, Keita"
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<[email protected]<mailto:[email protected]>>
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*Subject: *Re: [mpiwg-ft] [EXTERNAL] Re: FTWG Con Call Today
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Probably here since we don't have an issue for this discussion. If
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you want to open issues in our working group's repository
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(github.com/mpiwg-ft/ft-issues), that's probably fine.
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On December 20, 2016 at 3:47:25 PM, Teranishi, Keita
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([email protected]<mailto:[email protected]>
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<mailto:[email protected]>) wrote:
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Wesley,
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Should I do here or github issues?
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Thanks,
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Keita
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*From: *"Bland, Wesley" <[email protected]<mailto:[email protected]>>
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*Date: *Tuesday, December 20, 2016 at 1:43 PM
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*To: *MPI WG Fault Tolerance and Dynamic Process Control working
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Group <[email protected]<mailto:[email protected]>>, "Teranishi, Keita"
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<[email protected]<mailto:[email protected]>>
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*Subject: *Re: [mpiwg-ft] [EXTERNAL] Re: FTWG Con Call Today
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You don't have to wait. :) If you have comments/concerns, you can
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raise them here too.
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On December 20, 2016 at 3:38:47 PM, Teranishi, Keita
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([email protected]<mailto:[email protected]> <mailto:[email protected]>) wrote:
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All,
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Sorry, I could not make it today.  I will definitely join the
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meeting next time to make comments/suggestions on the three
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items (ULFM, ULFM+Auto, and ReInit) from Fenix perspective.
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Thanks,
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Keita
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*From: *<[email protected]<mailto:[email protected]>> on behalf of
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"Bland, Wesley" <[email protected]<mailto:[email protected]>>
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*Reply-To: *MPI WG Fault Tolerance and Dynamic Process Control
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working Group <[email protected]<mailto:[email protected]>>
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*Date: *Tuesday, December 20, 2016 at 1:29 PM
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*To: *FTWG <[email protected]<mailto:[email protected]>>
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*Subject: *[EXTERNAL] Re: [mpiwg-ft] FTWG Con Call Today
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The notes from today's call are posted on the wiki:
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https://github.com/mpiwg-ft/ft-issues/wiki/2016-12-20
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Those who have specific items, please make progress on those
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between now and our next meeting. We will be cancelling the Jan
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3 call due to the holiday. The next call will be on Jan 17.
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Thanks,
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Wesley
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On December 20, 2016 at 8:15:06 AM, Bland, Wesley
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([email protected]<mailto:[email protected]> <mailto:[email protected]>) wrote:
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The Fault Tolerance Working Group¹s biweekly con call is
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today at 3:00 PM Eastern. Today's agenda:
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* Recap of face to face meeting
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* Go over existing tickets
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* Discuss concerns with ULFM and path forward
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Thanks,
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Wesley
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Re: [mpiwg-ft] [EXTERNAL] Re: FTWG Con Call Today

Van Der Wijngaart, Rob F