[Mpi3-ft] Choosing a BLANK or SHRINK model for the RTS proposal

Graham, Richard L. rlgraham at ornl.gov
Tue Jan 24 16:40:50 CST 2012

Comment in line:

On Jan 24, 2012, at 3:11 PM, Sur, Sayantan wrote:

>> -----Original Message-----
>> From: mpi3-ft-bounces at lists.mpi-forum.org [mailto:mpi3-ft-
>> bounces at lists.mpi-forum.org] On Behalf Of Graham, Richard L.
>> Sent: Tuesday, January 24, 2012 11:42 AM
>> To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group
>> Subject: Re: [Mpi3-ft] Choosing a BLANK or SHRINK model for the RTS
>> proposal
>> I will re-iterate what I said before.  While this may one mode that
>> apps may want to use, it is not the only mode.  In particular, this
>> forces all ranks in a communicator to know about the change, even if
>> they have implemented a "local" algorithm that does not need to know
>> about all failures.
> I guess we can talk about this tomorrow in the WG meeting. My question is the current RTS proposal also requires everyone to call MPI_Comm_validate and know about other failures if they want to use collectives.

If you are going to use a global operation, you must make sure all support it, and the validate has been defined as the function to do that.  If you don't want global operations, you don't need to do a global check.  Also, nothing forces one to call the validate function, but if one wants to know if the collective succeeded on all ranks, one has to call validate - or build the equivalent functionality at the app layer.


>> Rich
>> On Jan 24, 2012, at 2:09 PM, Sur, Sayantan wrote:
>> Hi Josh,
>> Thanks for the crisp characterization of the proposal I was making. It
>> is correct. I was naturally thinking of the SHRINK mode, since it
>> involves least number of changes to MPI standard itself. Folks at the
>> forum also had similar thoughts (e.g. why does MPI_Comm_size() still
>> return count including failed procs).
>> Cf. http://www.netlib.org/utk/people/JackDongarra/PAPERS/isc2004-FT-
>> MPI.pdf
>> In this communicator mode, the ranks of MPI processes before and after
>> recovery might change, as well as the size of MPI COMM WORLD does
>> change. The appealing part of this communicator mode however is, that
>> all functions specified in MPI-1 and MPI-2 are still valid without any
>> further modification, since groups and communicators do not have wholes
>> (sic) and blank processes."
>> We can discuss further tomorrow as to whether we could go with SHRINK
>> mode (simplifying the proposal). From what I read in the paper, they
>> report being able to convert fault-tolerant master/worker to use both
>> modes.
>> 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> [mailto:mpi3-ft-bounces at lists.mpi-
>> forum.org] On Behalf Of Josh Hursey
>> Sent: Tuesday, January 24, 2012 10:39 AM
>> To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group
>> Subject: [Mpi3-ft] Choosing a BLANK or SHRINK model for the RTS
>> proposal
>> First let me say that I greatly appreciate the effort of Sayantan and
>> others to push us towards considering alternative techniques, and
>> stimulating discussion about design decisions. This is exactly the type
>> of discussion that needs to occur, and the working group is the most
>> appropriate place to have it.
>> One of the core suggestions of Sayantan's proposal is the switch from
>> (using FT-MPI's language) a model like BLANK to a model like SHRINK. I
>> think many of the other semantics are derived from this core shift, so
>> we should probably focus the discussion on this point earlier in our
>> conversation.
>> The current RTS proposal allows for a communicator to contain failed
>> processes and continue to be used for all operations, including
>> collectives, after acknowledging them. This matches closely to FT-MPI's
>> BLANK mode. The user can use MPI_Comm_split() to get the equivalent of
>> SHRINK if they need it.
>> The suggested modification allows for only/primarily a SHRINK-like mode
>> in order to have full functionality in the communicator. As discussed
>> on the previous call, one can get the BLANK mode by adding a library on
>> top of MPI that virtualizes the communicators to create shadow
>> communicators. The argument for the SHRINK mode is that it is -easier-
>> to pass/explain.
>> The reason we chose BLANK was derived from the literature reviewed,
>> code examples available, and feedback from application groups. >From
>> which there seemed to be a strong demand for the BLANK mode. In fact, I
>> had a difficult time finding good use cases for the SHIRNK mode (I'm
>> still looking though). Additionally, a BLANK mode seems also to make it
>> easier to reason about process recovery. To reason about process
>> recovery (something like FT-MPI's REBUILD mode) one needs to be able to
>> reason about the missing processes without changing the identities of
>> the existing processes, which can be difficult in a SHRINK mode. So
>> from this review it seemed that there was an application demand for a
>> BLANK-like mode for the RTS proposal.
>> In light of this background, it is concerning to me to advise these
>> application users that MPI will not provide the functionality they
>> require, but they have to depend upon a non-standard, third-party
>> library because we shied away from doing the right thing by them. This
>> background is advised from my review of the state of the art, but
>> others may have alternative evidence/commentary to present as well that
>> could sway the discussion. It just seems like a weak argument that we
>> should do the easy thing at the expense of doing the right thing by the
>> application community.
>> I certainly meant this email to stimulate conversation for the
>> teleconference tomorrow. In particular, I would like those on the list
>> with experience building ABFT/Natural FT applications/libraries (UTK?)
>> to express their perspective on this topic. Hopefully they can help
>> guide us towards the right solution, which might just be a SHRINK-like
>> mode.
>> -- Josh
>> --
>> Joshua Hursey
>> Postdoctoral Research Associate
>> Oak Ridge National Laboratory
>> http://users.nccs.gov/~jjhursey
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