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

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Date:   Sun, 20 Aug 2017 08:00:25 +0200
From:   Mike Galbraith <efault@....de>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@....com>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggeman <dietmar.eggemann@....com>,
        kernel-team@...roid.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] sched/fair: Make PELT signal more accurate

On Sat, 2017-08-19 at 10:58 -0700, Joel Fernandes wrote:
> 
> > Ok, I gotta ask:  In order to fix what?  What exactly does the small
> > but existent overhead increase buy us other than an ever so slightly
> > different chart?  What is your motivation to care about a microscopic
> > change in signal shape?
> 
> I wouldn't call the change "microscopic", its about 2% absolute which
> comes down to 1% with this change (if you count in relative terms, its
> higher and you can see the bump as the signal rises).
> If you look at the first chart at [1] at 3.74, that's not microscopic
> at all to me.

Whether that 0.02->0.01 is viewed as significant by you or I matters
not at all, for it to be significant, it must have measurable effect. 

Where is the consumer which benefits from precision improvement.. of
the instantaneously wildly inaccurate average.  Where is the non-zero
return on investment?  If the chart is the entire product, no sale.

	-Mike