Re: [RFC] CPU hard limits

Avi Kivity
2009-06-05T04:46:23Z

Balbir Singh wrote:
> On Fri, Jun 5, 2009 at 11:33 AM, Avi Kivity <avi@redhat.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.

--
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