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From: John Alvord <jalvo@mbay.net>
To: Steinar Hauan <hauan@cmu.edu>
Cc: <linux-kernel@vger.kernel.org>
Subject: Re: smp cputime issues (patch request ?)
Date: Fri, 04 Jan 2002 21:51:12 -0800
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On Sat, 5 Jan 2002 00:21:44 -0500 (EST), Steinar Hauan <hauan@cmu.edu>
wrote:

>On Thu, 3 Jan 2002, J.A. Magallon wrote:
>> 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.
>
>very correct.
>
>> 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.
>
>one of the tips received was to set the penalty for cpu switch, i.e. set
>
>  linux/include/asm/smp.h:#define PROC_CHANGE_PENALTY   15
>
>to a much higher value (50). this had no effect on the results.
>
>> 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.
>
>this would be an issue, agreed, but cache invalidation by cpu bounces
>should also affect one-cpu jobs? thus is does not explain why this
>effect should be (much) worse with 2 jobs.

One factor to consider is that to see it bounce, you need to be
running an observation process like top, or if it is a GUI display two
processes (application and X). Those observing processes will
continuosly bump aside the calcuation processes, causing a bouncing
effect.

john
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