fair group scheduler not so fair?

From: Chris Friesen
Date: Wed May 21 2008 - 20:00:04 EST


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.

Thanks,

Chris
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