Re: [RFC] CPU hard limits

Balbir Singh
2009-06-05T05:20:32Z

* Avi Kivity <avi@redhat.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