linux-kernel - Re: [patch 00/18] CFS Bandwidth Control v7.2

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Message-ID: <4E730397.5030902@google.com>
Date:	Fri, 16 Sep 2011 01:06:47 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <bsingharora@...il.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelianov <xemul@...allels.com>,
	Jason Baron <jbaron@...hat.com>
Subject: Re: [patch 00/18] CFS Bandwidth Control v7.2

Hi Vladimir,

I had a fairly good coversation with Pavel at LPC regarding these 
questions, it's probably worth syncing up with him and then following up 
if you still have questions.


On 09/13/11 05:10, Vladimir Davydov wrote:
> Hello, Paul
>
> I have a question about CFS bandwidth control.
>
> Let's consider a cgroup with several (>1) tasks running on a two CPU
> host. Let the limit of the cgroup be 50% (e.g. period=1s, quota=0.5s).
> How will tasks of the cgroup be distributed between the two CPUs? Will
> they all run on one of the CPUs, or will one half of them run on one CPU
> and others run on the other?
>

Parallelism is unconstrained until the bandwidth limit is reached, at 
which point we CONFIG_NR_CPUS=0

> Although in both cases the tasks will consume not more than one half of
> overall CPU time, the first case (all tasks of the cgroup run on the
> same CPU) is obviously better if the tasks are likely to communicate
> with each other (e.g. through pipe) which is often the case when cgroups
> are used for container virtualization.
>

This case is handled already by the affine wake-up path.

> In other words, I'd like to know if your code (or the scheduler code)
> tries to gather all tasks of the same cgroup on such a subset of all
> CPUs so that the tasks can't execute less CPUs without losing quota
> during each period. And if not, are you going to address the issue?
>

Parallelism != Bandwidth; no plans at this time.

Thanks!

- Paul
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