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

[Peter Zijlstra]

On Wed, Mar 02, 2016 at 11:49:48PM +0100, Rafael J. Wysocki wrote:
> >>> +       min_f = sg_policy->policy->cpuinfo.min_freq;
> >>> +       max_f = sg_policy->policy->cpuinfo.max_freq;
> >>> +       next_f = util > max ? max_f : min_f + util * (max_f - min_f) / max;

> In case a more formal derivation of this formula is needed, it is
> based on the following 3 assumptions:
> 
> (1) Performance is a linear function of frequency.
> (2) Required performance is a linear function of the utilization ratio
> x = util/max as provided by the scheduler (0 <= x <= 1).

> (3) The minimum possible frequency (min_freq) corresponds to x = 0 and
> the maximum possible frequency (max_freq) corresponds to x = 1.
> 
> (1) and (2) combined imply that
> 
> f = a * x + b
> 
> (f - frequency, a, b - constants to be determined) and then (3) quite
> trivially leads to b = min_freq and a = max_freq - min_freq.

3 is the problem, that just doesn't make sense and is probably the
reason why you see very little selection of the min freq.

Suppose a machine with the following frequencies:

	500, 750, 1000

And a utilization of 0.4, how does asking for 500 + 0.4 * (1000-500) =
700 make any sense? Per your point 1, it should should be asking for
0.4 * 1000 = 400.

Because, per 1, at 500 it runs exactly half as fast as at 1000, and we
only need 0.4 times as much. Therefore 500 is more than sufficient.



Note. we all know that 1 is a 'broken' assumption, but lacking anything
better I think its a reasonable one to make.