Re: CPU affinity & IPI latency

[David Lang]

A real-world example of this issue.

I was gzipping a large (~800MB) file on a dual athlon box. the gzip prcess
was bouncing back and forth between the two CPUs. I actually was able to
gzip faster by starting up setiathome to keep one CPU busy and force the
scheduler to keep the gzip on a single CPU (I ran things several times to
verify it was actually faster)

David Lang

 On Fri, 13 Jul 2001, Mike Kravetz wrote:

> Date: Fri, 13 Jul 2001 10:05:21 -0700
> From: Mike Kravetz <[e-mail]>
> To: Larry McVoy <[e-mail]>
> Cc: Davide Libenzi <[e-mail]>, [e-mail],
>      Andi Kleen <[e-mail]>, [e-mail]
> Subject: Re: CPU affinity & IPI latency
>
> On Thu, Jul 12, 2001 at 05:36:41PM -0700, Larry McVoy wrote:
> > Be careful tuning for LMbench (says the author :-)
> >
> > Especially this benchmark.  It's certainly possible to get dramatically better
> > SMP numbers by pinning all the lat_ctx processes to a single CPU, because
> > the benchmark is single threaded.  In other words, if we have 5 processes,
> > call them A, B, C, D, and E, then the benchmark is passing a token from
> > A to B to C to D to E and around again.
> >
> > If the amount of data/instructions needed by all 5 processes fits in the
> > cache and you pin all the processes to the same CPU you'll get much
> > better performance than simply letting them float.
> >
> > But making the system do that naively is a bad idea.
>
> I agree, and can't imagine the system ever attempting to take this
> into account and leave these 5 tasks on the same CPU.
>
> At the other extreme is my observation that 2 tasks on an 8 CPU
> system are 'round robined' among all 8 CPUs.  I think having the
> 2 tasks stay on 2 of the 8 CPUs would be an improvement with respect
> to CPU affinity.  Actually, the scheduler does 'try' to do this.
>
> It is clear that the behavior of lat_ctx bypasses almost all of
> the scheduler's attempts at CPU affinity.  The real question is,
> "How often in running 'real workloads' are the schduler's attempts
> at CPU affinity bypassed?".
>
> --
> Mike Kravetz                                 [e-mail]
> IBM Linux Technology Center
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