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

[Richard Treumann]

This proposal is not a minor change.

Please do not make this hole in the standard and assume you can later add 
language to standardize everything that comes through the hole.

If the standard is to introduce the notion of a recoverable error it must 
be as part of a full description of what "recovery" means. 

I think is is dangerous and ultimately useless to have implementors mark a 
failure as "recoverable" when the post error state of the distributed MPI 
has gone from "fully standards compliant" to "mostly standards compliant, 
read my user doc read my legal disclaimer, cross your fingers".

See comment below for why I do not think the new hole is needed to allow 
people to do implementation specific recoverability.

There is not even anything to prevent on implementation from deciding to 
add a function MPXX_WHAT_STILL_WORKS(err_code, answer) and documenting 5 
or 5000 enumerated values for "answer" ranging from NOTHING through 
TAKE_A_CHANCE_IF_YOU_LIKE to  EVERYTHING.

IBM would probably return TAKE_A_CHANCE_IF_YOU_LIKE because I cannot 
imagine how we would promise exactly what will work and what will not but 
in practice most things will still work as expected. 




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


mpi3-ft-bounces at lists.mpi-forum.org wrote on 09/21/2010 04:54:08 PM:

> [image removed] 
> 
> Re: [Mpi3-ft] Defining the state of MPI after an error
> 
> Bronis R. de Supinski 
> 
> to:
> 
> MPI 3.0 Fault Tolerance and Dynamic Process Control working Group 
> 
> 09/21/2010 04:59 PM
> 
> Sent by:
> 
> mpi3-ft-bounces at lists.mpi-forum.org
> 
> Please respond to "Bronis R. de Supinski", "MPI 3.0 Fault Tolerance 
> and Dynamic Process Control working Group" 
> 
> 
> Dick:
> 
> Re:
> > The current MPI standard does not say the MPI implementation is 
totally 
> > broken once there is an error.  Saying MPI state is undefined after an 

> > error simply says that the detailed semantic of the MPI standard can 
no 
> > longer be promised. In other words, after an error you leave behind 
the 
> > security of a portable standard semantic.  You are operating at your 
own 
> > risk. You do not need to read more than that into it.
> 
> Perhaps my problem with this position is that I come from the
> background of language definitions for compilers. When you
> read "undefined" in the OpenMP specification then you are
> being told that things are broken and the implementation does
> need to do anything or even tell you what they actually do (and
> I believe the same is true for the C and C++ standards). An
> alternative is "implementation defined", which requires the
> implementer to document what they actually do. Without that,
> you cannot even rely on actions with a specific implementation
> (unless you believe "My tests so far have not failed so I am OK").


When a standard says behavior is "undefined" in some situation, it cannot 
mean behavior is "broken". It cannot mean the implementor is prohibited 
from making it still work. It cannot mean the implementor is prohibited 
from making certain things work and documenting them. Any statement like 
this in a standard would be definition of behavior and the behavior would 
no longer be "undefined". 

The only thing a standard can logically mean by "undefined" is that the 
STANDARD no longer mandates the definition. 

Bronis says:

> 
> I strongly feel "undefined" should be reserved for situations that
> mean "your program is irrevocably broken and the implementer does
> not need to worry about what happens to it after encountering them."

I would say this as:

I strongly feel "undefined" should be reserved for situations that mean 
"The standard no longer guarantees your program is not irrevocably broken. 
The implementer is not required by the standard to worry about what 
happens to it after encountering them. An Implementation is free to 
provide any "better" behavior that may be of value but users cannot assume 
another implementation provides similar behavior so cannot assume 
standards defined portability."

I do not see how the use if the word "undefined" in a standard can be 
interpreted as a prohibition of any behavior an implementation might 
offer.




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