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

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Message-ID: <20090605044946.GA11755@balbir.in.ibm.com>
Date:	Fri, 5 Jun 2009 12:49:46 +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 07:44:27]:

> 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.

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?


>> 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?
>
Ignore this part of my comment

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

True.

-- 
	Balbir
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