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

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Date:	Sun, 07 Jun 2009 09:04:49 +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 09:01:50AM +0300, Avi Kivity wrote:
>   
>> Bharata B Rao wrote:
>>     
>>> 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 agree that guarantees are important, but I am not sure about
>
> 1. specifying both limits and guarantees for groups and
>   

Why would you allow specifying a lower bound for cpu usage (a 
guarantee), and upper bound (a limit), but not both?

> 2. not deriving guarantees from limits.
>
> Guarantees are met by some form of throttling or limiting and hence I think
> limiting should drive the guarantees

That would be fine if it didn't idle the cpu despite there being demand 
and available cpu power.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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