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Subject: Re: [OMPI users] High CPU usage with yield_when_idle =1 on CFS
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2011-09-01 22:06:36

On 8/31/2011 11:48 PM, Randolph Pullen wrote:
> I recall a discussion some time ago about yield, the Completely F%’d
> Scheduler (CFS) and OpenMPI.
> My system is currently suffering from massive CPU use while busy
> waiting. This gets worse as I try to bump up user concurrency.
> I am running with yield_when_idle but its not enough.
> Is there anything else I can do to release some CPU resource?
> I recall seeing one post where usleep(1) was inserted around the
> yields, is this still feasible?
> I'm using 1.4.1 - is there a fix to be found in upgrading?
> Unfortunately I am stuck with the CFS as I need Linux. Currently its
> Ubuntu 10.10 with
I think OMPI doesn't yet do (much/any) better than what you've
observed. You might be able to hack something up yourself. In
something I did recently, I replaced blocking sends and receives with
test/nanosleep loops. An "optimum" solution (minimum latency, optimal
performance at arbitrary levels of under and oversubscription) might be
elusive, but hopefully you'll quickly be able to piece together
something for your particular purposes. In my case, I was lucky and the
results were very gratifying... my bottleneck vaporized for modest
levels of oversubscription (2-4 more processes than processors).