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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