<br><font size=2 face="sans-serif">Rich </font>
<br>
<br><font size=2 face="sans-serif">I am not on the FT working group. I
do not know the ins and out of your ultimate goals or how you hope they
can be accomplished. I do not intend to become immersed in the complete
FT specification. I cannot afford the time.</font>
<br>
<br><font size=2 face="sans-serif">Those who are immersed seem to feel
they have a "minor" tweak to add to the standard before they
have the rest of FT worked out. It does not sound "minor"
to me but maybe I have misunderstood some part of it.</font>
<br>
<br><font size=2 face="sans-serif">I think the logical thing for me to
do is wait for the ticket. To be voted, the ticket must completely specify
the functionality and include the precise text to explain what it means.
If, by the time you have a ticket, my concerns are resolved by what
it says, I will not object.</font>
<br>
<br><font size=2 face="sans-serif">If I am still concerned by what the
ticket proposes, I must lobby against it and at that point I can stick
to the merits or risks contained in the ticket language only.</font>
<br>
<br><font size=2 face="sans-serif">At this point, I am trying to discuss
the shape of a cloud of smoke and the discussion has become unproductive.
I have tried several ways of making my concern understandable so
I do not know what to make of the question "</font><tt><font size=2>What
are your objections here</font></tt><font size=2 face="sans-serif">?"
at this stage.</font>
<br>
<br><font size=2 face="sans-serif">I understand that something like MPI_ERR_CANNOT_CONTINUE
could possibly make sense in the context of a complete FT chapter that
devotes tens of pages to saying exactly when it is appropriate to return
and how it is to be understood by people who wish to write portable MPI
applications.</font>
<br>
<br><font size=2 face="sans-serif">
Dick</font>
<br>
<br><font size=2 face="sans-serif">Dick Treumann - MPI Team
<br>
IBM Systems & Technology Group<br>
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601<br>
Tele (845) 433-7846 Fax (845) 433-8363<br>
</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">From:</font>
<td><font size=1 face="sans-serif">"Graham, Richard L." <rlgraham@ornl.gov></font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">To:</font>
<td><font size=1 face="sans-serif">"MPI 3.0 Fault Tolerance and Dynamic
Process Control working Group" <mpi3-ft@lists.mpi-forum.org></font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Date:</font>
<td><font size=1 face="sans-serif">09/22/2010 08:52 PM</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Subject:</font>
<td><font size=1 face="sans-serif">Re: [Mpi3-ft] Defining the state of
MPI after an error</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Sent by:</font>
<td><font size=1 face="sans-serif">mpi3-ft-bounces@lists.mpi-forum.org</font></table>
<br>
<hr noshade>
<br>
<br>
<br><tt><font size=2>Dick,<br>
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.<br>
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.<br>
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.<br>
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.<br>
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.<br>
<br>
Thanks,<br>
Rich<br>
<br>
On 9/22/10 6:55 PM, "Richard Treumann" <treumann@us.ibm.com>
wrote:<br>
<br>
<br>
We are kind of going in circles because the context and rationale for CANNOT_CONTINUE
are still too ambiguous.<br>
<br>
My argument is against adding it into the standard first and figuring out
later what it means.<br>
<br>
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.<br>
<br>
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..<br>
<br>
Note<br>
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.<br>
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)<br>
<br>
<br>
Dick Treumann - MPI Team<br>
IBM Systems & Technology Group<br>
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601<br>
Tele (845) 433-7846 Fax (845) 433-8363<br>
<br>
<br>
<br>
From: Darius Buntinas <buntinas@mcs.anl.gov><br>
To: "MPI 3.0 Fault Tolerance and Dynamic Process Control working Group"
<mpi3-ft@lists.mpi-forum.org><br>
Date: 09/22/2010 05:47 PM<br>
Subject: Re: [Mpi3-ft] Defining the state of MPI after an error<br>
Sent by: mpi3-ft-bounces@lists.mpi-forum.org<br>
________________________________<br>
<br>
<br>
<br>
<br>
On Sep 22, 2010, at 2:29 PM, Richard Treumann wrote:<br>
<br>
><br>
> 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"?<br>
<br>
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 :-)<br>
<br>
> 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?<br>
<br>
I don't know yet. It might be something as simple as adding an entry
to the error class table with a description like:<br>
<br>
Process can no longer perform any MPI operations. If
an MPI operation<br>
returns this error class, all subsequent calls to MPI functions
will<br>
return this error class.<br>
<br>
> 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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
So we either get a hang, or everyone gets a CANNOT_CONTINUE but we avoided
the discombobulated scenario.<br>
<br>
-d<br>
<br>
<br>
<br>
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