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

[Peter Zijlstra]

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?

> 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?