linux-kernel - Re: [RFC] CPU hard limits

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Message-ID: <20090605092733.GA27486@in.ibm.com>
Date:	Fri, 5 Jun 2009 14:57:33 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
Cc:	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>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Herbert Poetzl <herbert@...hfloor.at>
Subject: Re: [RFC] CPU hard limits

On Fri, Jun 05, 2009 at 01:53:15AM -0700, Paul Menage wrote:
> On Wed, Jun 3, 2009 at 10:36 PM, Bharata B
> Rao<bharata@...ux.vnet.ibm.com> wrote:
> > - Hard limits can be used to provide guarantees.
> >
> 
> This claim (and the subsequent long thread it generated on how limits
> can provide guarantees) confused me a bit.
> 
> Why do we need limits to provide guarantees when we can already
> provide guarantees via shares?

shares design is proportional and hence it can't by itself provide
guarantees.

> 
> Suppose 10 cgroups each want 10% of the machine's CPU. We can just
> give each cgroup an equal share, and they're guaranteed 10% if they
> try to use it; if they don't use it, other cgroups can get access to
> the idle cycles.

Now if 11th group with same shares comes in, then each group will now
get 9% of CPU and that 10% guarantee breaks.

Regards,
Bharata.
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