linux-kernel - Re: [PATCH] sched: fair: Improve PELT decay

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Message-ID: <20170322170204.hacokajtuabrjlia@hirez.programming.kicks-ass.net>
Date:   Wed, 22 Mar 2017 18:02:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched: fair: Improve PELT decay_load calculation comments

On Wed, Mar 22, 2017 at 09:35:43AM -0700, Joel Fernandes wrote:
> On Wed, Mar 22, 2017 at 7:16 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Fri, Mar 10, 2017 at 12:23:41PM -0800, Joel Fernandes wrote:
> >> The PELT decay_load comments are a bit confusing, first of all
> >> the 1/2^N should be (1/2)^N so that the reader doesn't get confused.
> >
> > I'm thinking you're confused. They're identical.
> >
> > (1/2)^N = (2^-1)^N = 2^-N = 1/2^N
> 
> They are identical I know, but I meant by enclosing the 1/2 in
> brackets, it is more clear that we multiply by 1/2 N times to the
> first time reader - for the reason that we'd like to reduce the PELT
> calculated load by 1/2 N times.

Must be me then, because I've never been confused about that. Esp. so
since the first part: y^p = 1/2, explicitly mentions half. So its clear
from the factorization that half is meant.

> >> Secondly, the y^N splitting into a 2-part decay factor deserves
> >> a better explanation. This patch improves the comments.
> >
> > I find its actually harder to read.
> 
> Oh, which part? Can you help improve it? Maybe I didn't word something
> correctly?

I think the fact that there's now words actually makes it worse.

The equation very concisely shows what we do. I don't see why we need
extra words there to obscure things.