Re: fair group scheduler not so fair?
From: Chris Friesen
Date: Mon Jun 02 2008 - 16:03:28 EST
Srivatsa Vaddagiri wrote:
That seems to be pretty difficult to achieve with the per-cpu runqueue
and smpnice based load balancing approach we have now.
Okay, thanks.
Initially I tried a simple setup with three hogs all in the default "sys"
group. Over multiple retries using 10-sec intervals, sometimes it gave
roughly 67% for each task, other times it settled into a 100/50/50 split
that remained stable over time.
Was this with imbalance_pct set to 105? Does it make any difference if
you change imbalance_pct to say 102?
It was set to 105 initially. I later reproduced the problem with 102.
For example, the following was with 102, with three tasks created in the
sys class. Based on the runtime, pid 2499 has been getting a cpu all to
itself for over a minute.
2499 cfriesen 20 0 3800 392 336 R 99.8 0.0 1:05.85 cat
2496 cfriesen 20 0 3800 392 336 R 50.0 0.0 0:32.95 cat
2498 cfriesen 20 0 3800 392 336 R 50.0 0.0 0:32.97 cat
The next run was much better, with sub-second fairness after a minute.
2505 cfriesen 20 0 3800 392 336 R 68.2 0.0 1:00.32 cat
2506 cfriesen 20 0 3800 392 336 R 66.9 0.0 0:59.85 cat
2503 cfriesen 20 0 3800 392 336 R 64.2 0.0 1:00.21 cat
The lack of predictability is disturbing, as it implies some sensitivity
to the specific test conditions.
With three groups, one task in each, I tried both 10 and 60 second
intervals. The longer interval looked better but was still up to 0.8% off:
I honestly don't know if we can do better than 0.8%! In any case, I'd
expect that it would require more drastic changes.
No problem. It's still far superior than the SMP performance of CKRM,
which is what we're currently using (although heavily modified).
Chris
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