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

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Message-ID: <4A291753.7090205@redhat.com>
Date:	Fri, 05 Jun 2009 16:02:11 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Paul Menage <menage@...gle.com>
CC:	bharata@...ux.vnet.ibm.com, 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>, kvm@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Herbert Poetzl <herbert@...hfloor.at>
Subject: Re: [RFC] CPU hard limits

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?
>
> 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.
>
> Suppose cgroup A wants a guarantee of 50% and two others, B and C,
> want guarantees of 15% each; give A 50 shares and B and C 15 shares
> each. In this case, if they all run flat out they'll get 62%/19%/19%,
> which is within their SLA.
>
> That's not to say that hard limits can't be useful in their own right
> - e.g. for providing reproducible loadtesting conditions by
> controlling how much CPU a service can use during the load test. But I
> don't see why using them to implement guarantees is either necessary
> or desirable.
>
> (Unless I'm missing some crucial point ...)
>   

How many shares does a cgroup with a 0% guarantee get?

Ideally, the scheduler would hand out cpu time according to weight and 
demand, then clamp over-demand by a cgroup's limit and boost the share 
to meet guarantees.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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