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

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Date:	Sun, 07 Jun 2009 09:09:01 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Balbir Singh <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:
>> 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%.
>>     
>
> Excellent examples, we've covered them in the RFC, could you see if we
> missed anything in terms of use cases? The real question is do we care
> enough to build hard limits control into the CFS group scheduler. I
> believe we should.
>   

You only cover the limit part.  Guarantees are left as an exercise to 
the reader.

I don't think implementing guarantees via limits is workable as it 
causes the cpu to be idled unnecessarily.

>>> 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.
>>     
>
> To be honest, I would disagree here, specifically if you start
> comparing how you would build guarantees in the kernel and compare it
> with the proposed approach. I don't want to harp on the technicality,
> but point out the feasibility for people who care for lower end of the
> guarantee without requiring density. I think the real technical
> discussion should be on here are the use cases, lets agree on the need
> for the feature and go ahead and start prototyping the feature.
>   

I don't understand.  Are you saying implementing guarantees is too complex?

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

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