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

[Richard Treumann]

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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