Messages in this thread | | | From | John Alvord <> | 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.
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