Messages in this thread | | | Subject | Re: fair group scheduler not so fair? | From | Peter Zijlstra <> | Date | Thu, 22 May 2008 08:56:57 +0200 |
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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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