Re: [Mpi3-ft] Stabilization Updated & MPI_Comm_size question
We do have a way to tell if they "died with out a peep" do this defined in the proposed spec, but the question is does it matter. If it happens right after MPI_Init() and apps have not carried out any MPI related setup, it probably does not matter to the app. However, I am not sure how we would be able to tell, unless we declare that the first call to MPI_Comm_size() is the defining moment - seems a bit risky to me. Rich On 9/17/10 10:55 AM, "Bronis R. de Supinski" <[email protected]> wrote: Dick: How could you tell whether they never hatched or merely died without a peep? Bronis On Fri, 17 Sep 2010, Richard Treumann wrote:
If I used some mechanism that allows the MPI launch to give me fewer processes than I asked for and it does give me less than I asked for, what does this mean?
"number of ranks in the communicator regardless of whether they're operational"
It is sort of weird to say that the processes I was never given are "in the communicator". I can see the logic in counting a processes that have died as "ranks in the communicator"
I would not count chickens that never hatched.
Dick Treumann - MPI Team IBM Systems & Technology Group Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 Tele (845) 433-7846 Fax (845) 433-8363
From: "Bronevetsky, Greg" <[email protected]> To: "Bronis R. de Supinski" <[email protected]>, "MPI 3.0 Fault Tolerance and Dynamic Process Control working Group" <[email protected]> Date: 09/17/2010 10:03 AM Subject: Re: [Mpi3-ft] Stabilization Updated & MPI_Comm_size question Sent by: [email protected]
________________________________
I agree that MPI_Comm_size should return the number of ranks in the communicator regardless of whether they're operational or not. However, this just pushes the question further back: if the user asked for N processes but only M have started up, how many ranks should MPI_COMM_WORLD have? Either choice is going to be self-consistent from MPI's point of view. If its N, then some ranks will be dead. If it is M then the application may not have enough processes to work with. The former (N) case has property that the user doesn't need to add code to check for this condition since their existing error checking code will catch this situation. The latter case (M) is nice because it cheaper to check whether we got fewer processes than expected than to explicitly try to communicate with them.
As such, I don't see a strong motivation for choosing either. However, we should just pick one and stick with it to avoid unnecessary API divergence.
Greg Bronevetsky Lawrence Livermore National Lab (925) 424-5756 [email protected] http://BLOCKEDgreg.bronevetsky.com<http://BLOCKEDgreg.bronevetsky.com/>
-----Original Message----- From: [email protected] [mailto:mpi3-ft- [email protected]] On Behalf Of Bronis R. de Supinski Sent: Friday, September 17, 2010 5:23 AM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] Stabilization Updated & MPI_Comm_size question
I agree with Rich and Darius.
On Fri, 17 Sep 2010, Darius Buntinas wrote:
I don't think we should change the standard in this case. For
MPI_Comm_size to have any useful meaning, it needs to return the size of the communicator: i.e., if comm_size returns N, you should be able to do a send to processes 0 through N-1. Of course if some of those processes have failed, you'll get an error associated with the process failure, but never an error for an invalid rank.
As discussed in the section about mpiexec, an implementation may decide
to provide a soft process count argument. So "mpiexec -n 10 -soft 5 ./cpi" can start any number between 5 and 10 processes. But that does not affect the meaning of the size of MPI_COMM_WORLD: regardless of the number of processes the implementation decides to start, MPI_Comm_size will return the _actual_ number of processes started.
-d
On Sep 17, 2010, at 11:22 AM, Graham, Richard L. wrote:
We need to clearly define what N or M is and not leave it to the
implementation. 100% of the codes that seen over the past 15 years that check this value use it to indicate how many process have started. Any thing else is really useless, aside from letting the user find out how many processes actually started up, and then know how many did not start up.
Rich
On 9/17/10 4:27 AM, "Josh Hursey" <[email protected]> wrote:
So the Run-Through Stabilization proposal has been updated per our
discussion in the working group meeting at the MPI Forum. The changes are summarized below:
- Add a Legacy Library Support example - Clarify new error classes - Update the MPI_Init and MPI_Finalize wording to be simpler and more direct. - Fix wording of group creation calls versus communicator creation calls.
https://BLOCKEDBLOCKEDBLOCKEDsvn.mpi-forum.org/trac/mpi-forum-<https://BLOCKEDblockedblockedsvn.mpi-forum.org/trac/mpi-forum-> web/wiki/ft/run_through_stabilization
One question that we discussed quite a bit during the meeting was the issue of the return value of MPI_Comm_size() when processes fail during launch. I attempted to capture the discussion in the room in the Open Question attached to the discussion of MPI_Init: https://BLOCKEDBLOCKEDBLOCKEDsvn.mpi-forum.org/trac/mpi-forum-<https://BLOCKEDblockedblockedsvn.mpi-forum.org/trac/mpi-forum-> web/wiki/ft/run_through_stabilization#MPI_INIT
Open question: If the user asks to start N processes on the command line, and only M processes were successfully launched (where M < N), then what should be returned from MPI_COMM_SIZE?
The return value must be consistent across all alive members of the group. The issue is if it should return N or M.
The feeling in the room was that since the MPI standard does not define the ability for the user to ask for a specific number of processes before initthen it is hard to define that this is the number it should be.
So it is left to the implementation whether it is M or N. If it is M, then the user has other techniques to find out what it originally asked for (e.g., by passing it as a command line argument to the application itself).
What do people think about the MPI_Comm_size issue?
-- Josh
------------------------------------ Joshua Hursey Postdoctoral Research Associate Oak Ridge National Laboratory http://BLOCKEDBLOCKEDBLOCKEDwww.BLOCKEDBLOCKEDBLOCKEDcs.indiana.edu/~jjhursey<http://BLOCKEDblockedblockedwww.BLOCKEDblockedblockedcs.indiana.edu/~jjhursey>
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participants (1)
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Graham, Richard L.