linux-kernel - Re: [PATCH v4 1/4] sched/fair: Introduce primitives for CFS bandwidth burst

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Message-ID: <YFNtesvbxUY4XTYR@hirez.programming.kicks-ass.net>
Date:   Thu, 18 Mar 2021 16:10:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     changhuaixin <changhuaixin@...ux.alibaba.com>,
        Benjamin Segall <bsegall@...gle.com>, dietmar.eggemann@....com,
        juri.lelli@...hat.com, khlebnikov@...dex-team.ru,
        open list <linux-kernel@...r.kernel.org>, mgorman@...e.de,
        mingo@...hat.com, Odin Ugedal <odin@...d.al>,
        Odin Ugedal <odin@...dal.com>, Paul Turner <pjt@...gle.com>,
        rostedt@...dmis.org, Shanpei Chen <shanpeic@...ux.alibaba.com>,
        Tejun Heo <tj@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        xiyou.wangcong@...il.com
Subject: Re: [PATCH v4 1/4] sched/fair: Introduce primitives for CFS
 bandwidth burst

On Thu, Mar 18, 2021 at 08:59:44AM -0400, Phil Auld wrote:
> I admit to not having followed all the history of this patch set. That
> said, when I see the above I just think your quota is too low for your
> workload.

This.

> The burst (mis?)feature seems to be a way to bypass the quota.  And it
> sort of assumes cooperative containers that will only burst when they
> need it and then go back to normal. 

Its not entirely unreasonable or unheard of. There's soft realtime
systems that use this to increase the utilization with the trade-off
that you're going to miss deadlines once every so often.

If you do it right, you can calculate the probabilities. Or usually the
other way around, you calculate the allowed variance/burst given a P
value for making the deadline.

Input then isn't the WCET for each task, but a runtime distribution as
measured for your workload on your system etc..

I used to have papers on this, but I can't seem to find them in a hurry.