linux-kernel - Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

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Message-ID: <20230910182023.hdrhx6b5bq5r4vfo@airbuntu>
Date:   Sun, 10 Sep 2023 19:20:23 +0100
From:   Qais Yousef <qyousef@...alina.io>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

On 09/07/23 13:53, Peter Zijlstra wrote:
> On Thu, Sep 07, 2023 at 08:48:08AM +0100, Lukasz Luba wrote:
> 
> > > Hehe. That's because they're not really periodic ;-)
> > 
> > They are periodic in a sense, they wake up every 16ms, but sometimes
> > they have more work. It depends what is currently going in the game
> > and/or sometimes the data locality (might not be in cache).
> > 
> > Although, that's for games, other workloads like youtube play or this
> > one 'Yahoo browser' (from your example) are more 'predictable' (after
> > the start up period). And I really like the potential energy saving
> > there :)
> 
> So everything media is fundamentally periodic, you're hard tied to the
> framerate / audio-buffer size etc..
> 
> Also note that the traditional periodic task model from the real-time
> community has the notion of WCET, which completely covers this
> fluctuation in frame-to-frame work, it only considers the absolute worst
> case.
> 
> Now, practically, that stinks, esp. when you care about batteries, but
> it does not mean these tasks are not periodic.

piecewise periodic?

> Many extentions to the periodic task model are possible, including
> things like average runtime with bursts etc.. all have their trade-offs.

The challenge we have is the endless number of workloads we need to cater for..
Or you think one of these models can actually scale to that?


Thanks!

--
Qais Yousef