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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Mar 3, 2016 at 9:06 PM, Steve Muckle <steve.muckle@linaro.org> wrote:
> 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.

So as I said, the formula I used didn't take invariance into account,
so that's quite as expected.

> 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.

I guess you mean performance in terms of the overhead?

Thanks,
Rafael