Re: smp cputime issues (patch request ?)

From: Davide Libenzi (
Date: Wed Jan 02 2002 - 21:39:10 EST

On Thu, 3 Jan 2002, J.A. Magallon wrote:

> On 20020102 Steinar Hauan wrote:
> >hello,
> >
> > we are encountering some weird timing behaviour on our linux cluster.
> >
> > specifically: when running 2 copies of selected programs on a
> > dual-cpu system, the cputime reported for each process is up to 25%
> > higher than when the processes are run on their own. however, if running
> > two different jobs on the same machine, both complete with a cputime
> > equal to when run individually. sample timing output attached.
> >
> Cache pollution problems ?
> As I understand, your job does not use too much memory, does no IO,
> just linear algebra (ie, matrix-times-vector or vector-plus-vector
> operations). That implies sequential access to matrix rows and vectors.
> I will try to guess...
> Problem with linux scheduler is that processes are bounced from one CPU
> to the other, they are not tied to one, nor try to stay in the one they
> start, even if there is no need for the cpu to do any other job.
> On an UP box, the cache is useful to speed up your matrix-vector ops.
> One process on a 2-way box, just bounces from one cpu to the other,
> and both caches are filled with the same data. Two processes on two
> cpus, and everytime they 'swap' between cpus they trash the previous
> cache for the other job, so when it returs it has no data cached.
> Solutions:
> - cpu affinity patch: manually tie processes to cpus
> - new scheduler: a patch for the scheduler that tries to
> keep processes on the cpu they start was talked about on the list.
> I would prefer the second option. I think it is named something like
> 'multiqueue scheduler', and its 'father' could be (AFAIR) Davide Libezni.
> Look for that on the list archives. Problem: I think the patch only
> exists for 2.5.

The patch is here :

I did not read the whole thread but if your two tasks are strictly cpu
bound and you've two cpus, you should not have problems even with the
current scheduler.

- Davide

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