Re: [PATCH v2] sched/fair: Make PELT signal more accurate

[Mike Galbraith]

On Fri, 2017-08-18 at 16:50 -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 high - as much 2% in my testing. On plotting
> signals, I found that there are errors especially high when we update just
> before the period boundary is hit. These errors can be significantly reduced if
> we update the averages more often.
> 
> 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 512us).

Ok, I gotta ask: ÂIn order to fix what? ÂWhat exactly does the small
but existent overhead increase buy us other than an ever so slightly
different chart? ÂWhat is your motivation to care about a microscopic
change in signal shape?

	-Mike