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

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Message-ID: <20090605052014.GD11755@balbir.in.ibm.com>
Date:	Fri, 5 Jun 2009 13:20:14 +0800
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.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

* Avi Kivity <avi@...hat.com> [2009-06-05 08:16:21]:

> 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.

Avi,

Could you look at my newer email and comment, where I've mentioned
that I see your concern and discussed a design point. We could
probably take this discussion forward from there?

-- 
	Balbir
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