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

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Message-ID: <20090605134320.GA3994@linux.vnet.ibm.com>
Date:	Fri, 5 Jun 2009 19:13:20 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Paul Menage <menage@...gle.com>, bharata@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org,
	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

On Fri, Jun 05, 2009 at 04:02:11PM +0300, Avi Kivity wrote:
> 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?
>

Shares cannot be used to provide guarantees. All they decide is what
propotion groups can get CPU time. (yes, shares is a bad name, weight
shows the intent better).

thanks,
-- 
regards,
Dhaval
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