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

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Date:	Fri, 05 Jun 2009 08:16:21 +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:

    

>>> How, it works out fine in my calculation
>>>
>>> 50 + 40 for G2 and G3, make sure that G1 gets 10%, since others are
>>> limited to 90%
>>> 50 + 40 for G1 and G3, make sure that G2 gets 10%, since others are
>>> limited to 90%
>>> 50 + 50 for G1 and G2, make sure that G3 gets 0%, since others are
>>> limited to 100%
>>>   
>>>       
>> It's fine in that it satisfies the guarantees, but it is deeply  
>> suboptimal.  If I ran a cpu hog in the first group, while the other two  
>> were idle, it would be limited to 50% cpu.  On the other hand, if it  
>> consumed all 100% cpu it would still satisfy the guarantees (as the  
>> other groups are idle).
>>
>> The result is that in such a situation, wall clock time would double  
>> even though cpu resources are available.
>>     
>
> 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?
>   

Suppose in my example cgroup 1 consumed 100% of the cpu resources and 
cgroup 2 and 3 were completely idle.  All of the guarantees are met (if 
cgroup 2 is idle, there's no need to give it the 10% cpu time it is 
guaranteed).

If  your only tool to achieve the guarantees is a limit system, then 
yes, the equation yields the correct results.  But given that it yields 
such inferior results, I think we need to look for a more involved solution.

I think the limits method fits cases where it is difficult to evict a 
resource (say, disk quotas -- if you want to guarantee 10% of space to 
cgroups 1, you must limit all others to 90%).  But for processor usage, 
you can evict a cgroup instantly, so nothing prevents a cgroup from 
consuming all available resources as long as others do not contend for them.

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

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