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Subject: Re: [OMPI users] Openmpi and processor affinity
From: Ralph Castain (rhc_at_[hidden])
Date: 2009-06-03 12:36:59

The changes Jeff mentioned are not in the 1.3 branch - not sure if they will
come over there or not.

I'm a little concerned in this thread that someone is reporting the process
affinity binding changing - that shouldn't be happening, and my guess is
that something outside of our control may be changing it.

One other thing to consider that has been an issue around here, and will be
an even bigger issue with the change to bind at app start. If your app is
threaded, we will bind *all* threads to the same processor, thus potentially
hampering performance. We have found that multi-threaded apps often provide
better performance if users do *not* set processor affinity via MPI, but
instead embed binding calls inside the individual threads so they can be
placed on separate processors.

All depends on the exact nature of the application, of course!


On Wed, Jun 3, 2009 at 10:02 AM, Jeff Squyres <jsquyres_at_[hidden]> wrote:

> On Jun 3, 2009, at 11:40 AM, Ashley Pittman wrote:
> Wasn't there a discussion about this recently on the list, OMPI binds
>> during MPI_Init() so it's possible for memory to be allocated on the
>> wrong quad, the discussion was about moving the binding to the orte
>> process as I recall?
> Yes. It's been fixed in OMPI devel trunk. I'm not sure it made it to the
> v1.3 branch, but it's definitely not in a released version yet.
> I *thought* that HPL did all allocation after MPI_INIT. But I could be
> wrong. If so, then using numactl to bind before the MPI app starts will
> likely give better results -- you're right (until we get our fixes in such
> that we bind pre-main).
> Regardless, if something is *changing* the affinity after MPI_INIT, then
> there's little OMPI can do about that.
> >From my testing of process affinity you tend to get much more consistent
>> results with it on and much more unpredictable results with it off, I'd
>> questing that it's working properly if you are seeing a 88-93% range in
>> the results.
>> Ashley Pittman.
>> _______________________________________________
>> users mailing list
>> users_at_[hidden]
> --
> Jeff Squyres
> Cisco Systems
> _______________________________________________
> users mailing list
> users_at_[hidden]