Messages in this thread | | | Date | Mon, 7 Aug 2017 15:40:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/fair: Make PELT signal more accurate |
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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?
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