Re: [RFC] CPU hard limits

Balbir Singh
2009-06-05T04:50:06Z

* Avi Kivity <avi@redhat.com> [2009-06-05 07:44:27]:

> 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.

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