Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data

Steve Muckle
2016-03-03T20:06:46Z

On 03/03/2016 05:07 AM, Vincent Guittot wrote:
> I mainly want to prevent any useless and periodic frequency switch
> because of an utilization that changes with the current frequency (if
> frequency invariance is not used) and that can make the formula
> selects another frequency than the current one. That what i can see
> when testing it .
>
> Sorry for the late reply, i was trying to do some test on my board but
> was facing some crash issue (not link with your patchset). So i have
> done some tests and i can see such instable behavior. I have generated
> a load of 33% at max frequency (3ms runs every 9ms) and i can see the
> frequency that toggles without any good reason. Saying that, i can see
> similar thing with ondemand.

FWIW I ran some performance numbers on my chromebook 2. Initially I
forgot to bring in the frequency invariance support but that yielded an
opportunity to see the impact of it.

The tests below consist of a periodic workload. The OH (overhead)
numbers show how close the workload got to running as slow as fmin (100%
= as slow as powersave gov, 0% = as fast as perf gov). The OR (overrun)
number is the count of instances where the busy work exceeded the period.

First a comparison of schedutil with and without frequency invariance.
Run and period are in milliseconds.

scu (no inv) scu (w/inv)
run period busy % OR OH OR OH
1 100 1.00% 0 79.72% 0 95.86%
10 1000 1.00% 0 24.52% 0 71.61%
1 10 10.00% 0 21.25% 0 41.78%
10 100 10.00% 0 26.06% 0 47.96%
100 1000 10.00% 0 6.36% 0 26.03%
6 33 18.18% 0 15.67% 0 31.61%
66 333 19.82% 0 8.94% 0 29.46%
4 10 40.00% 0 6.26% 0 12.93%
40 100 40.00% 0 6.93% 2 14.08%
400 1000 40.00% 0 1.65% 0 11.58%
5 9 55.56% 0 3.70% 0 7.70%
50 90 55.56% 1 4.19% 6 8.06%
500 900 55.56% 0 1.35% 5 6.94%
9 12 75.00% 0 1.60% 56 3.59%
90 120 75.00% 0 1.88% 21 3.94%
900 1200 75.00% 0 0.73% 4 4.41%

Frequency invariance causes schedutil overhead to increase noticeably. I
haven't dug into traces or anything. Perhaps this is due to the
algorithm overshooting then overcorrecting etc., I do not yet know.

Here is a comparison, with frequency invariance, of ondemand and
interactive with schedfreq and schedutil. The first two columns (run and
period) are omitted so the table will fit.

ondemand interactive schedfreq schedutil
busy % OR OH OR OH OR OH OR OH
1.00% 0 68.96% 0 100.04% 0 78.49% 0 95.86%
1.00% 0 25.04% 0 22.59% 0 72.56% 0 71.61%
10.00% 0 21.75% 0 63.08% 0 52.40% 0 41.78%
10.00% 0 12.17% 0 14.41% 0 17.33% 0 47.96%
10.00% 0 2.57% 0 2.17% 0 0.29% 0 26.03%
18.18% 0 12.39% 0 9.39% 0 17.34% 0 31.61%
19.82% 0 3.74% 0 3.42% 0 12.26% 0 29.46%
40.00% 2 6.26% 1 12.23% 0 6.15% 0 12.93%
40.00% 0 0.47% 0 0.05% 0 2.68% 2 14.08%
40.00% 0 0.60% 0 0.50% 0 1.22% 0 11.58%
55.56% 2 4.25% 5 5.97% 0 2.51% 0 7.70%
55.56% 0 1.89% 0 0.04% 0 1.71% 6 8.06%
55.56% 0 0.50% 0 0.47% 0 1.82% 5 6.94%
75.00% 2 1.65% 1 0.46% 0 0.26% 56 3.59%
75.00% 0 1.68% 0 0.05% 0 0.49% 21 3.94%
75.00% 0 0.28% 0 0.23% 0 0.62% 4 4.41%

Aside from the 2nd and 3rd tests schedutil is showing decreased
performance across the board. The fifth test is particularly bad.

The catch is that I do not have power numbers to go with this data, as
I'm not currently equipped to gather them. So more analysis is
definitely needed to capture the full story.

thanks,
Steve