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

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Message-ID: <CAJWu+oqU3dj1i0anSx68ahxZREzmdCvifF7A-CN855v2WFXuJA@mail.gmail.com>
Date:   Wed, 22 Mar 2017 12:19:17 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

Hi Peter,

On Wed, Mar 22, 2017 at 10:02 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.

Yes that's true.

>> >> 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.

Ok, I agree with you and will kill this patch then. Thanks for the review.

Regards,
Joel