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SubjectRe: fair group scheduler not so fair?
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On Wed, 2008-05-21 at 17:59 -0600, Chris Friesen wrote:
> I just downloaded the current git head and started playing with the fair
> group scheduler. (This is on a dual cpu Mac G5.)
>
> I created two groups, "a" and "b". Each of them was left with the
> default share of 1024.
>
> I created three cpu hogs by doing "cat /dev/zero > /dev/null". One hog
> (pid 2435) was put into group "a", while the other two were put into
> group "b".
>
> After giving them time to settle down, "top" showed the following:
>
> 2438 cfriesen 20 0 3800 392 336 R 99.5 0.0 4:02.82 cat
>
> 2435 cfriesen 20 0 3800 392 336 R 65.9 0.0 3:30.94 cat
>
> 2437 cfriesen 20 0 3800 392 336 R 34.3 0.0 3:14.89 cat
>
>
>
> Where pid 2435 should have gotten a whole cpu worth of time, it actually
> only got 66% of a cpu. Is this expected behaviour?
>
>
>
> I then redid the test with two hogs in one group and three hogs in the
> other group. Unfortunately, the cpu shares were not equally distributed
> within each group. Using a 10-sec interval in "top", I got the following:
>
>
> 2522 cfriesen 20 0 3800 392 336 R 52.2 0.0 1:33.38 cat
>
> 2523 cfriesen 20 0 3800 392 336 R 48.9 0.0 1:37.85 cat
>
> 2524 cfriesen 20 0 3800 392 336 R 37.0 0.0 1:23.22 cat
>
> 2525 cfriesen 20 0 3800 392 336 R 32.6 0.0 1:22.62 cat
>
> 2559 cfriesen 20 0 3800 392 336 R 28.7 0.0 0:24.30 cat
>
>
> Do we expect to see upwards of 9% relative unfairness between processes
> within a class?
>
> I tried messing with the tuneables in /proc/sys/kernel
> (sched_latency_ns, sched_migration_cost, sched_min_granularity_ns) but
> was unable to significantly improve these results.
>
> Any pointers would be appreciated.

What you're testing is SMP fairness of group scheduling and that code is
somewhat fresh (and has known issues - performance nr 1 amogst them) but
its quite possible it has some other issues as well.

Could you see if the patches found here:

http://programming.kicks-ass.net/kernel-patches/sched-smp-group-fixes/

make any difference for you?



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