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Date: Thu, 3 Mar 2016 17:24:32 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Linux PM list <linux-pm@...r.kernel.org>,
Juri Lelli <juri.lelli@....com>,
Steve Muckle <steve.muckle@...aro.org>,
ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
Viresh Kumar <viresh.kumar@...aro.org>,
Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler
utilization data
On Thu, Mar 3, 2016 at 1:20 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.
It is about mapping the entire [0,1] interval to the available frequency range.
I till overprovision things (the smaller x the more), but then it may
help the race-to-idle a bit in theory.
> 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.
OK, but then I don't see why this reasoning only applies to the lower
bound of the frequency range. Is there any reason why x = 1 should be
the only point mapping to max_freq?
If not, then I think it's reasonable to map the middle of the
available frequency range to x = 0.5 and then we have b = 0 and a =
(max_freq + min_freq) / 2.
I'll try that and see how it goes.
> Note. we all know that 1 is a 'broken' assumption, but lacking anything
> better I think its a reasonable one to make.
Right.
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