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

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Date:	Fri, 05 Jun 2009 07:44:27 +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:
> On Fri, Jun 5, 2009 at 11:33 AM, Avi Kivity <avi@...hat.com> wrote:
>   
>> Bharata B Rao wrote:
>>     
>>>> Another way is to place the 8 groups in a container group, and limit
>>>>  that to 80%. But that doesn't work if I want to provide guarantees to
>>>>  several groups.
>>>>
>>>>         
>>> Hmm why not ? Reduce the guarantee of the container group and provide
>>> the same to additional groups ?
>>>
>>>       
>> This method produces suboptimal results:
>>
>> $ cgroup-limits 10 10 0
>> [50.0, 50.0, 40.0]
>>
>> I want to provide two 10% guaranteed groups and one best-effort group.
>>  Using the limits method, no group can now use more than 50% of the
>> resources.  However, having the first group use 90% of the resources does
>> not violate any guarantees, but it not allowed by the solution.
>>
>>     
>
> 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.
> Now if we really have zeros, I would recommend using
>
> cgroup-limits 10 10 and you'll see that you'll get 90, 90 as output.
>
> Adding zeros to the calcuation is not recommended. Does that help?

What do you mean, it is not recommended? I have two groups which need at 
least 10% and one which does not need any guarantee, how do I express it?

In any case, changing the zero to 1% does not materially change the results.

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

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