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

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Message-ID: <4A28B4CE.4010004@redhat.com>
Date:	Fri, 05 Jun 2009 09:01:50 +0300
From:	Avi Kivity <avi@...hat.com>
To:	bharata@...ux.vnet.ibm.com
CC:	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...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

Bharata B Rao wrote:
> On Fri, Jun 05, 2009 at 01:27:55PM +0800, Balbir Singh wrote:
>   
>> * Avi Kivity <avi@...hat.com> [2009-06-05 08:21:43]:
>>
>>     
>>> Balbir Singh wrote:
>>>       
>>>>> But then there is no other way to make a *guarantee*, guarantees come
>>>>> at a cost of idling resources, no? Can you show me any other
>>>>> combination that will provide the guarantee and without idling the
>>>>> system for the specified guarantees?
>>>>>     
>>>>>           
>>>> OK, I see part of your concern, but I think we could do some
>>>> optimizations during design. For example if all groups have reached
>>>> their hard-limit and the system is idle, should we do start a new hard
>>>> limit interval and restart, so that idleness can be removed. Would
>>>> that be an acceptable design point?
>>>>         
>>> I think so.  Given guarantees G1..Gn (0 <= Gi <= 1; sum(Gi) <= 1), and a  
>>> cpu hog running in each group, how would the algorithm divide resources?
>>>
>>>       
>> As per the matrix calculation, but as soon as we reach an idle point,
>> we redistribute the b/w and start a new quantum so to speak, where all
>> groups are charged up to their hard limits.
>>     
>
> But could there be client models where you are required to strictly
> adhere to the limit within the bandwidth and not provide more (by advancing
> the bandwidth period) in the presence of idle cycles ?
>   

That's the limit part.  I'd like to be able to specify limits and 
guarantees on the same host and for the same groups; I don't think that 
works when you advance the bandwidth period.

I think we need to treat guarantees as first-class goals, not something 
derived from limits (in fact I think guarantees are more useful as they 
can be used to provide SLAs).

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

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