linux-kernel - Re: [RFC v2 PATCH 0/8] CFS Hard limits

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Date:	Wed, 30 Sep 2009 20:09:53 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Cc:	Pavel Emelyanov <xemul@...nvz.org>, linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>
Subject: Re: [RFC v2 PATCH 0/8] CFS Hard limits - v2

On Wed, Sep 30, 2009 at 07:55:37PM +0530, Bharata B Rao wrote:
> On Wed, Sep 30, 2009 at 05:36:29PM +0400, Pavel Emelyanov wrote:
> > Bharata B Rao wrote:
> > > Hi,
> > > 
> > > Here is the v2 post of hard limits feature for CFS group scheduler. This
> > > RFC post mainly adds runtime borrowing feature and has a new locking scheme
> > > to protect CFS runtime related fields.
> > > 
> > > It would be nice to have some comments on this set!
> > 
> > I have a question I'd like to ask before diving into the code.
> > Consider I'm a user, that has a 4CPUs box 2GHz each and I'd like
> > to create a container with 2CPUs 1GHz each. Can I achieve this
> > after your patches?
> 
> I am not sure if I understand the GHz specification you mention here.
> Are you saying that you want run a container with 2 CPUS with each of
> them running at half their (frequency)capacity ?
> 
> This hard limits scheme is about time based rate limiting where you can
> specify a runtime(=hard limit) and a period for the container and the
> container will not be allowed to consume more than the specified CPU time
> within a given period.

IMO Pavel's requirement can be met with a hard limit of 25%

	2 CPU of 1GHz = (1/2 x 4) (1/2 x 2) GHz CPUs
		      = 1/4 x 4 2GHz CPU
		      = 25% of (4 2GHz CPU)

IOW by hard-limiting a container thread to run just 0.5sec every sec on a 2GHz 
cpu, it is effectively making progress at the rate of 1GHz?

- vatsa
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