[Mpi3-ft] Defining the state of MPI after an error
Richard Treumann
treumann at us.ibm.com
Wed Sep 22 13:16:46 CDT 2010
Darius
I can imagine a few errors that I know will be harmless to MPI state. I
can make sure nobody can do any harm by passing an invalid communicator to
MPI_COMM_SIZE.
I cannot think of a detectable error that would return and leave that
thread of that process so totally broken that nothing in MPI will work
from then on. In a collective, there may be processes in which the thread
that called the CC never returns and that tread of the process is no
longer usable because it is hung. Other threads using other communicators
in the process with a hung thread may work perfectly.
Except for the very few cases where I know there was no damage (like a bad
comm on MPI_COMM_SIZE) the situation, 99.99% of the time, will be that
everything still works but sometimes the outcome is a surprise to the
user. Say you do:
1 MPI_Barrier (on world)
2 MPI_Barrier (on world):
3 other stuff
4 MPI_Barrier (on world)
5 if (my rank is even)
6 sendrecv(with odd neighbor)
7 else
8 sendrecv(with even neighbor)
but get back an error at all even numbered ranks from the line 1 barrier
call. The line 2 MPI_Barrier may still "work" but the line 2 barrier at
even numbered ranks will match the line 1 barrier at odd ranks. Even ranks
will begin "other stuff" and odd ranks will sit in the line 2 barrier
until even ranks finish "other stuff" and reach the line 4 barrier. The
odd ranks now get through their line 2 barrier and begin other stuff.
If "other stuff" involves communication among the even ranks and
communication among the odd ranks. that will work too. The even ranks will
all send/recv among themselves later the odd ranks will all send/recv
among themselves.
The even ranks will reach line 6 and hang there because the odd tasks are
still stuck at line 4.
In this entire example, libmpi has continued working "correctly" but the
behavior you get from correct behavior is not what you planned.
The situation of MPI state being totally trashed by an error that returns
a return code barely exists. The case where it is subtly discombobulated
is the norm.
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 12:24 PM
Subject:
Re: [Mpi3-ft] Defining the state of MPI after an error
Sent by:
mpi3-ft-bounces at lists.mpi-forum.org
OK, I (think I) see what you guys are saying, so maybe we should look at
it this way. The CANNOT_CONTINUE proposal should not define the operation
of the MPI implementation after errors other than CANNOT_CONTINUE.
Instead, it defines that after the implementation gives a CANNOT_CONTINUE
error, the app knows that the implementation is fatally wedged, and that
the user should definately not expect correct operation after this. I.e.,
we're not labeling other errors as "recoverable," we're just marking
CANNOT_CONTINUE as "unrecoverable."
Note that an implementation can still be standard compliant even if it
never returns a CANNOT_CONTINUE error even when it is fatally wedged
(because operation after any other error is still undefined).
This just defines a way for the implementation to let the user know that
it has given up. So that if the implementation provides best-effort
functionality after an error, and the user has "read the disclaimer" and
is comfortable with proceeding, this is a way to differentiate between an
error as a result of a failure that hosed everything, and one that may
allow things to continue.
We still would like to define what happens to a bcast after a process in
the communicator fails. But we leave that for future proposals.
Does this make sense?
-d
On Sep 22, 2010, at 8:43 AM, Terry Dontje wrote:
> Richard Treumann wrote:
>>
>> 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.
>>
> I think I agree with Dick on the above. Another way of putting the
disagreement is that Josh's proposal is too general in that not all
errorcodes can be completely marked as MPI state is broken or not. When
Sun implemented fault tolerant client/server we came up with a new error
class that when returned gave the user the understanding that a condition
occurred on a communicator that has rendered the communicator useless and
one should clean it up before continuing on. The point is there was a
concrete understanding of the error and what could be done to recover. As
opposed to a general class that say's everything is borked or not which
essential doesn't give you much because you'll end up eventually having to
define a more specific class of error IMO.
>
> --td
>>
>>
>>
>> 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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>>
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>>
>>
>>
>
>
> --
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> Terry D. Dontje | Principal Software Engineer
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> Oracle - Performance Technologies
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>
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