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From: George Bosilca (bosilca_at_[hidden])
Date: 2006-10-26 17:11:46

The Open MPI behavior is the same independently of the network used
for the job. At least the behavior dictated by our internal message
passing layer. But, for this to happens we should get a warning from
the network that something is wrong (such a timeout). In the case of
TCP (and Myrinet) the timeout is so high that Open MPI was not
informed that something went wrong (we printout some warnings when
this happens). It was happily waiting for a message to complete ...
Once the network cable was reconnected, the network device itself
recover and resume the communication, leading to a correct send
operation (and this without involving Open MPI at all). There is
nothing (that has a reasonable cost) we can do about this.

For IB, look like the network timeout is smaller. Open MPI knew that
something was wrong (the output prove it), and tried to continue
using the other available devices. If none are available, then Open
MPI is supposed to abort the job. For your particular run did you had
Ethernet between the nodes ? If yes, I'm quite sure the MPI run
wasn't stopped ... it continued using the TCP device (if not disabled
by hand at mpirun time).

That's not what is supposed to happens right now. If there are other
devices (such as TCP) the MPI job will print out some warnings and
will continue over the remaining networks (some will continue to use
the other networks, only the peer where the network went down get
affected). If the network timeout is too high, Open MPI will never
notice that something went wrong. At least not the default message
layer (PML).

If you want to have the job abort when your main network goes down,
disable the usage of the others available network. More specifically
disable the TCP. A simple way to do it, it's to add the following
argument to your mpirun command:

--mca btl ^tcp (or --mca btl opnib,sm,self).


PS: There are several internal message passing modules available for
Open MPI. The default one, look more for performance than
reliability. If reliability it's what you need you should use the DR
PML. For this, you can specify --mca pml dr at mpirun time. This (DR)
PML has data reliability and timeout (Open MPI internal timeout that
are configurable), allowing to recover faster from a network failure.

On Oct 26, 2006, at 3:52 PM, Troy Telford wrote:

> I've recently had the chance to see how Open MPI (as well as other
> MPIs)
> behave in the case of network failure.
> I've looked at what happens when a node has its network connection
> disconnected in the middle of a job, with Ethernet, Myrinet (GM), and
> InfiniBand (OpenIB).
> With Ethernet and Myrinet, the job more or less pauses until the
> cable is
> re-connected. (I imagine timeouts still apply, but I wasn't patient
> enough to wait for them)
> With InfiniBand, the job pauses and Open MPI throws a few error
> messages.
> After the cable is plugged back in (and the SM catches up), the job
> remains where it was when it was paused. I'd guess that part of
> this is
> that the timeout is much shorter with IB than with Myri or
> Ethernet, and
> that when I unplug the IB cable, it times out fairly quickly (and then
> Open MPI throws its error messages).
> At any rate, the thought occurs (and it may just be my ignorance of
> MPI):
> After a network connection times out (as was apparently the case
> with IB),
> is the job salvageable? If the jobs are not salvageable, why
> didn't Open
> MPI abort the job (and clean up the running processes on the nodes)?
> --
> Troy Telford
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