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

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Message-ID: <20201217133656.GX3040@hirez.programming.kicks-ass.net>
Date:   Thu, 17 Dec 2020 14:36:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Huaixin Chang <changhuaixin@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org, bsegall@...gle.com,
        dietmar.eggemann@....com, juri.lelli@...hat.com, mgorman@...e.de,
        mingo@...hat.com, pauld@...head.com, pjt@...gle.com,
        rostedt@...dmis.org, vincent.guittot@...aro.org,
        khlebnikov@...dex-team.ru, xiyou.wangcong@...il.com,
        shanpeic@...ux.alibaba.com
Subject: Re: [PATCH 1/4] sched/fair: Introduce primitives for CFS bandwidth
 burst

On Thu, Dec 17, 2020 at 03:46:17PM +0800, Huaixin Chang wrote:
> In this patch, we introduce the notion of CFS bandwidth burst. Unused
> "quota" from pervious "periods" might be accumulated and used in the
> following "periods". The maximum amount of accumulated bandwidth is
> bounded by "burst". And the maximun amount of CPU a group can consume in
> a given period is "buffer" which is equivalent to "quota" + "burst in
> case that this group has done enough accumulation.

Oh man, Juri, wasn't there a paper about statistical bandwidth
accounting somewhere? Where, if you replace every utilization by a
statistical variable, the end result is still useful?

That is, instead of something like; \Sum u_i <= 1, you get something
like: \Sum {avg(u),var(u)}_i <= {1, sqrt(\Sum var_i^2)} and you can
still proof bounded tardiness etc.. (assuming a gaussian distribution).

The proposed seems close to that, but not quite, and I'm afraid it's not
quite strong enough to still provide any guarantees.