Re: [PATCH] sched/fair: Make PELT signal more accurate

[Joel Fernandes]

Hi Peter,

On Mon, Aug 7, 2017 at 6:40 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Aug 04, 2017 at 08:40:23AM -0700, Joel Fernandes wrote:
>> The PELT signal (sa->load_avg and sa->util_avg) are not updated if the
>> amount accumulated during a single update doesn't cross a period
>> boundary.
>
>> This is fine in cases where the amount accrued is much smaller than
>> the size of a single PELT window (1ms) however if the amount accrued
>> is high then the relative error (calculated against what the actual
>> signal would be had we updated the averages) can be quite high - as
>> much 3-6% in my testing.
>
> The max accumulate we can have and not cross a boundary is 1023*1024 ns.
> At which point we get a divisor of LOAD_AVG_MAX - 1024 + 1023.
>
> So for util_sum we'd have a increase of 1023*1024/(47742-1) = ~22. Which
> on the total signal for util (1024) is ~2.1%
>
> Where does the 3-6% come from?

Sorry, I should have been more clear. This error (3-6%) I measured is
relative to what the signal could have been had we done the division.
Indeed I don't see any cases were the absolute error is more than ~22
/ 1024 as you mentioned.

>
>> Inorder to fix this, this patch does the average update by also
>> checking how much time has elapsed since the last update and update
>> the averages if it has been long enough (as a threshold I chose
>> 128us).
>
> This of course does the divisions more often; anything on performance
> impact?

Sure, I am working on those and will post an update soon.

Thanks!

-Joel


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