[Mpi3-ft] Defining the state of MPI after an error

[Graham, Richard L.]

Dick,
  What are your objections here ?  All the current proposal is doing is trying to define a set of consistent return codes, and is not changing anything about MPI.  I am not sure there is sufficient information to act on in all cases with the current error handling in MPI, but may be wrong on this.  However, there is nothing else that is being proposed at this stage.
  Also, if no changes are made to current implementations, there is a chance that apps will hang, but this is no different than if users set errors return today.  Which would need to do be done to see the effects of the changes.
  Now, it is fair to ask if this really adds something.  If there is intent to recover from errors, which is what the FT working group is trying to figure out, then this has a lot of value in that it is the venue for an initial discussion of how to extend the error classes, which is really what Josh has been trying to do.  I believe we need an implementation and some app experiments to find what we have missed.
  As for the choice of "MPI_ERR_CANNOT_CONTINUE" how is this any different than malloc returning null ?  It tells the user that the library is no longer functional, and leaves it to the app to decide how to respond.  Depending on the implementation, there are "error" scenarios that both the app and the MPI library can survive.  Failure of alloc_mem may be such a function.  An app may also decide that sending data to a specific destination may  also be ok - I can give a several real use cases that were brought to us as we were looking into this that would be just fine with this.  Now the collective operations are another question.
  So, all this proposal is really doing is start to revive the FT discussion at the Forum level, as partial implementation is getting to a state that it can be evaluated.  This is really why it is important to understand the specific shortcomings you see in what is being proposed - just the error propagation issues.

Thanks,
Rich

On 9/22/10 6:55 PM, "Richard Treumann" <treumann at us.ibm.com> wrote:


We are kind of going in circles because the context and rationale for CANNOT_CONTINUE are still too ambiguous.

My argument is against adding it into the standard first and figuring out later what it means.

I will wait for the ticket. If the ticket gives a full and convincing specification of what the implementor and the user are to do with it,, I will make my judgement based on the whole description.

If the ticket says "Put this minor change in today and we will decide later what it means, I must lobby the Forum to reject the ticket..

Note
1)  all current errors detected by an MPI application map to an existing error class. An error cannot map to two error classes so if some user error handler is presently checking for MPI_ERR_OP after a non-SUCCESS return from MPI_Reduce and the implementation moves the return code for passing a bad OP from class MPI_ERR_OP to MPI_ERR_CANNOT_CONTINUE it has just broken a user code.
2) Mandating that every MPI call after a MPI_ERR_CANNOT_CONTINUE must return MPI_ERR_CANNOT_CONTINUE will require that every MPI call check a  global flag (resulting in overhead and possible displacement of other data from cache)


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: Darius Buntinas <buntinas at mcs.anl.gov>
To: "MPI 3.0 Fault Tolerance and Dynamic Process Control working Group" <mpi3-ft at lists.mpi-forum.org>
Date: 09/22/2010 05:47 PM
Subject: Re: [Mpi3-ft] Defining the state of MPI after an error
Sent by: mpi3-ft-bounces at lists.mpi-forum.org
________________________________




On Sep 22, 2010, at 2:29 PM, Richard Treumann wrote:

>
> You lost me there - in part, i am saying it is useless because there are almost zero cases in which it would be appropriate.  How does that make it "a minor change"?

Well I figure we're just adding an error class that the implementation can return to the user if it gives up and can't continue.  That's minor.  Whether or not it's useful is another story :-)

> Can you provide me the precise text you would add to the standard? Exactly how does the CANNOT_CONTINUE work?  Under what conditions does an MPI process see a CANNOT_CONTINUE and what does it mean?

I don't know yet.  It might be something as simple as adding an entry to the error class table with a description like:

    Process can no longer perform any MPI operations.  If an MPI operation
    returns this error class, all subsequent calls to MPI functions will
    return this error class.

> Please look at the example again.  The point was that there is nothing there that would justify a CANNOT_CONTINUE and MPI is still working correctly. Despite that, the behavior is a mess from the algorithm viewpoint after the error.

Since we haven't defined what happens in a failed collective yet, consider an implementation could will not continue after a failed collective.  The odd numbered processes that did not immediately return from barrier with an error will continue with the barrier protocol (say it's recursive doubling).  Some of the odd processes will need to send messages to some of the even processes.  Upon receiving these messages, the even processes will respond with an I_QUIT message, or perhaps the connection is closed, so the odd processes will get a communication error when trying to send the message.  In either case, the odd processes will notice that something's wrong with the other processes, and return an error.  The second barrier will return a CANNOT_CONTINUE on all of the processes.

OK, what if the odd processes can't determine that the even processes can't continue?  The odd processes would hang in the first barrier, and the even numbered processes would get a CANNOT_CONTINUE from the second barrier.

So we either get a hang, or everyone gets a CANNOT_CONTINUE but we avoided the discombobulated scenario.

-d



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