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SubjectRe: smp cputime issues (patch request ?)
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.

Steinar Hauan, dept of ChemE --
Carnegie Mellon University, Pittsburgh PA, USA

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