Messages in this thread | | | Date | Sat, 5 Jan 2002 00:21:44 -0500 (EST) | From | Steinar Hauan <> | Subject | Re: smp cputime issues (patch request ?) |
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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.
regards, -- Steinar Hauan, dept of ChemE -- hauan@cmu.edu Carnegie Mellon University, Pittsburgh PA, USA
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