Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 17 Mar 2021 09:06:28 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     changhuaixin <changhuaixin@linux.alibaba.com>
Cc:     Benjamin Segall <bsegall@google.com>, dietmar.eggemann@arm.com,
        juri.lelli@redhat.com, khlebnikov@yandex-team.ru,
        open list <linux-kernel@vger.kernel.org>, mgorman@suse.de,
        mingo@redhat.com, Odin Ugedal <odin@uged.al>,
        Odin Ugedal <odin@ugedal.com>, pauld@redhead.com,
        Paul Turner <pjt@google.com>, rostedt@goodmis.org,
        Shanpei Chen <shanpeic@linux.alibaba.com>,
        Tejun Heo <tj@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        xiyou.wangcong@gmail.com
Subject: Re: [PATCH v4 1/4] sched/fair: Introduce primitives for CFS
 bandwidth burst
Message-ID: <YFG4hEOe65cbCo26@hirez.programming.kicks-ass.net>
References: <20210316044931.39733-1-changhuaixin@linux.alibaba.com>
 <20210316044931.39733-2-changhuaixin@linux.alibaba.com>
 <YFCAXeZj6sXBI5Ls@hirez.programming.kicks-ass.net>
 <B75EDF95-96B3-44E4-8169-3C1FCBC30A7B@linux.alibaba.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <B75EDF95-96B3-44E4-8169-3C1FCBC30A7B@linux.alibaba.com>
Precedence: bulk

On Wed, Mar 17, 2021 at 03:16:18PM +0800, changhuaixin wrote:

> > Why do you allow such a large burst? I would expect something like:
> > 
> > 	if (burst > quote)
> > 		return -EINVAL;
> > 
> > That limits the variance in the system. Allowing super long bursts seems
> > to defeat the entire purpose of bandwidth control.
> 
> I understand your concern. Surely large burst value might allow super
> long bursts thus preventing bandwidth control entirely for a long
> time.
> 
> However, I am afraid it is hard to decide what the maximum burst
> should be from the bandwidth control mechanism itself. Allowing some
> burst to the maximum of quota is helpful, but not enough. There are
> cases where workloads are bursty that they need many times more than
> quota in a single period. In such cases, limiting burst to the maximum
> of quota fails to meet the needs.
> 
> Thus, I wonder whether is it acceptable to leave the maximum burst to
> users. If the desired behavior is to allow some burst, configure burst
> accordingly. If that is causing variance, use share or other fairness
> mechanism. And if fairness mechanism still fails to coordinate, do not
> use burst maybe.

It's not fairness, bandwidth control is about isolation, and burst
introduces interference.

> In this way, cfs_b->buffer can be removed while cfs_b->max_overrun is
> still needed maybe.

So what is the typical avg,stdev,max and mode for the workloads where you find
you need this?

I would really like to put a limit on the burst. IMO a workload that has
a burst many times longer than the quota is plain broken.