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

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Date:	Fri, 05 Jun 2009 16:14:23 +0300
From:	Avi Kivity <avi@...hat.com>
To:	balbir@...ux.vnet.ibm.com
CC:	bharata@...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

Balbir Singh wrote:
>> 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.
>>     
>
> Yes, this feature needs to be configurable. But your use case for both
> limits and guarantees is interesting. We spoke to Peter and he was
> convinced only of the guarantee use case. Could you please help
> elaborate your use case, so that we can incorporate it into RFC v2 we
> send out. Peter is opposed to having hard limits and is convinced that
> they are not generally useful, so far I seen you and Paul say it is
> useful, any arguments you have or any +1 from you will help us. Peter
> I am not back stabbing you :)
>   

I am selling virtual private servers.  A 10% cpu share costs $x/month, 
and I guarantee you'll get that 10%, or your money back.  On the other 
hand, I want to limit cpu usage to that 10% (maybe a little more) so 
people don't buy 10% shares and use 100% on my underutilized servers.  
If they want 100%, let them pay for 100%.

>> 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).
>>     
>
> Even limits are useful for SLA's since your b/w available changes
> quite drastically as we add or remove groups. There are other use
> cases for limits as well

SLAs are specified in terms of guarantees on a service, not on limits on 
others.  If we could use limits to provide guarantees, that would be 
fine, but it doesn't quite work out.

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

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