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SubjectRe: fair group scheduler not so fair?
On Thu, May 29, 2008 at 03:30:37PM -0600, Chris Friesen wrote:
> Overall the group scheduler results look better, but I'm seeing an odd
> scenario within a single group where sometimes I get a 67/67/66 breakdown
> but sometimes it gives 100/50/50.

Hmm ..I cant recreate this 100/50/50 situation (tried about 10 times).

> Also, although the long-term results are good, the shorter-term fairness
> isn't great. Is there a tuneable that would allow for a tradeoff between
> performance and fairness?

The tuneables I can think of are:

- HZ (higher the better)
- min/max_interval and imbalance_pct for each domain (lower the better)

> I have people that are looking for within 4% fairness over a 1sec interval.

That seems to be pretty difficult to achieve with the per-cpu runqueue
and smpnice based load balancing approach we have now.

> 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?

> 3 tasks in sys
> 2471 cfriesen 20 0 3800 392 336 R 99.9 0.0 0:29.97 cat
> 2470 cfriesen 20 0 3800 392 336 R 50.3 0.0 0:17.83 cat
> 2469 cfriesen 20 0 3800 392 336 R 49.6 0.0 0:17.96 cat
>
> retry
> 2475 cfriesen 20 0 3800 392 336 R 68.3 0.0 0:28.46 cat
> 2476 cfriesen 20 0 3800 392 336 R 67.3 0.0 0:28.24 cat
> 2474 cfriesen 20 0 3800 392 336 R 64.3 0.0 0:28.73 cat
>
> 2476 cfriesen 20 0 3800 392 336 R 67.1 0.0 0:41.79 cat
> 2474 cfriesen 20 0 3800 392 336 R 66.6 0.0 0:41.96 cat
> 2475 cfriesen 20 0 3800 392 336 R 66.1 0.0 0:41.67 cat
>
> retry
> 2490 cfriesen 20 0 3800 392 336 R 99.7 0.0 0:22.23 cat
> 2489 cfriesen 20 0 3800 392 336 R 49.9 0.0 0:21.02 cat
> 2491 cfriesen 20 0 3800 392 336 R 49.9 0.0 0:13.94 cat
>
>
> 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.

> 10-sec
> 2490 cfriesen 20 0 3800 392 336 R 68.9 0.0 1:35.13 cat
> 2491 cfriesen 20 0 3800 392 336 R 65.8 0.0 1:04.65 cat
> 2489 cfriesen 20 0 3800 392 336 R 64.5 0.0 1:26.48 cat
>
> 60-sec
> 2490 cfriesen 20 0 3800 392 336 R 67.5 0.0 3:19.85 cat
> 2491 cfriesen 20 0 3800 392 336 R 66.3 0.0 2:48.93 cat
> 2489 cfriesen 20 0 3800 392 336 R 66.2 0.0 3:10.86 cat
>
>
> Finally, a more complicated scenario. three tasks in A, two in B, and one
> in C. The 60-sec trial was up to 0.8 off, while a 3-second trial (just for
> fun) was 8.5% off.
>
> 60-sec
> 2491 cfriesen 20 0 3800 392 336 R 65.9 0.0 5:06.69 cat
> 2499 cfriesen 20 0 3800 392 336 R 33.6 0.0 0:55.35 cat
> 2490 cfriesen 20 0 3800 392 336 R 33.5 0.0 4:47.94 cat
> 2497 cfriesen 20 0 3800 392 336 R 22.6 0.0 0:38.76 cat
> 2489 cfriesen 20 0 3800 392 336 R 22.2 0.0 4:28.03 cat
> 2498 cfriesen 20 0 3800 392 336 R 22.2 0.0 0:35.13 cat
>
> 3-sec
> 2491 cfriesen 20 0 3800 392 336 R 58.2 0.0 13:29.60 cat
> 2490 cfriesen 20 0 3800 392 336 R 34.8 0.0 9:07.73 cat
> 2499 cfriesen 20 0 3800 392 336 R 31.0 0.0 5:15.69 cat
> 2497 cfriesen 20 0 3800 392 336 R 29.4 0.0 3:37.25 cat
> 2489 cfriesen 20 0 3800 392 336 R 23.3 0.0 7:26.25 cat
> 2498 cfriesen 20 0 3800 392 336 R 23.0 0.0 3:33.24 cat

I ran with this configuration:

HZ = 1000,
min/max_interval = 1
imbalance_pct = 102

My 10-sec fairness looks like below (Error = 1.5%):

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ #C COMMAND
4549 root 20 0 1384 228 176 R 65.2 0.0 0:36.02 0 hogc
4547 root 20 0 1384 228 176 R 32.8 0.0 0:17.87 0 hogb
4548 root 20 0 1384 228 176 R 32.6 0.0 0:18.28 1 hogb
4546 root 20 0 1384 232 176 R 22.9 0.0 0:11.82 1 hoga
4545 root 20 0 1384 228 176 R 22.3 0.0 0:11.74 1 hoga
4544 root 20 0 1384 232 176 R 22.1 0.0 0:11.93 1 hoga

3-sec fairness (error = 2.3% ..sometimes went upto 6.7%)

4549 root 20 0 1384 228 176 R 69.0 0.0 1:33.56 1 hogc
4548 root 20 0 1384 228 176 R 32.7 0.0 0:46.74 1 hogb
4547 root 20 0 1384 228 176 R 29.3 0.0 0:47.16 0 hogb
4546 root 20 0 1384 232 176 R 22.3 0.0 0:30.80 0 hoga
4544 root 20 0 1384 232 176 R 20.3 0.0 0:30.95 0 hoga
4545 root 20 0 1384 228 176 R 19.4 0.0 0:31.17 0 hoga

--
Regards,
vatsa


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