Messages in this thread | | | Date | Wed, 2 Jan 2002 18:39:10 -0800 (PST) | From | Davide Libenzi <> | Subject | Re: smp cputime issues (patch request ?) |
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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 :
http://www.xmailserver.org/linux-patches/xsched-2.5.2-pre4-0.58.diff
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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