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Date:   Thu, 25 Apr 2019 19:44:25 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thara Gopinath <thara.gopinath@...aro.org>, mingo@...hat.com,
        rui.zhang@...el.com, linux-kernel@...r.kernel.org,
        amit.kachhap@...il.com, viresh.kumar@...aro.org,
        javi.merino@...nel.org, edubezval@...il.com,
        daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
        nicolas.dechesne@...aro.org, bjorn.andersson@...aro.org,
        dietmar.eggemann@....com, Quentin Perret <quentin.perret@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH V2 0/3] Introduce Thermal Pressure


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Apr 17, 2019 at 08:29:32PM +0200, Ingo Molnar wrote:
> > > Assuming PeterZ & Rafael & Quentin doesn't hate the whole thermal load 
> > > tracking approach. 
> > 
> > I seem to remember competing proposals, and have forgotten everything
> > about them; the cover letter also didn't have references to them or
> > mention them in any way.
> > 
> > As to the averaging and period, I personally prefer a PELT signal with
> > the windows lined up, if that really is too short a window, then a PELT
> > like signal with a natural multiple of the PELT period would make sense,
> > such that the windows still line up nicely.
> > 
> > Mixing different averaging methods and non-aligned windows just makes me
> > uncomfortable.
> 
> Yeah, so the problem with PELT is that while it nicely approximates 
> variable-period decay calculations with plain additions, shifts and table 
> lookups (i.e. accelerates pow()), AFAICS the most important decay 
> parameter is fixed: the speed of decay, the dampening factor, which is 
> fixed at 32:
> 
>   Documentation/scheduler/sched-pelt.c
> 
>   #define HALFLIFE 32
> 
> Right?
> 
> Thara's numbers suggest that there's high sensitivity to the speed of 
> decay. By using PELT we'd be using whatever averaging speed there is 
> within PELT.
> 
> Now we could make that parametric of course, but that would both 
> complicate the PELT lookup code (one more dimension) and would negatively 
> affect code generation in a number of places.

I missed the other solution, which is what you suggested: by 
increasing/reducing the PELT window size we can effectively shift decay 
speed and use just a single lookup table.

I.e. instead of the fixed period size of 1024 in accumulate_sum(), use 
decay_load() directly but use a different (longer) window size from 1024 
usecs to calculate 'periods', and make it a multiple of 1024.

This might just work out right: with a half-life of 32 the fastest decay 
speed should be around ~20 msecs (?) - and Thara's numbers so far suggest 
that the sweet spot averaging is significantly longer, at a couple of 
hundred millisecs.

Thanks,

	Ingo

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