[Devel] Re: [RFC] CPU hard limits

[Paul Menage]

On Fri, Jun 5, 2009 at 2:48 AM, Dhaval Giani<dhaval at linux.vnet.ibm.com> wrote:
>> > Now if 11th group with same shares comes in, then each group will now
>> > get 9% of CPU and that 10% guarantee breaks.
>>
>> So you're trying to guarantee 11 cgroups that they can each get 10% of
>> the CPU? That's called over-committing, and while there's nothing
>> wrong with doing that if you're confident that they'll not all need
>> their 10% at the same time, there's no way to *guarantee* them all
>> 10%. You can guarantee them all 9% and hope the extra 1% is spare for
>> those that need it (over-committing), or you can guarantee 10 of them
>> 10% and give the last one 0 shares.
>>
>> How would you propose to guarantee 11 cgroups each 10% of the CPU
>> using hard limits?
>>
>
> You cannot guarantee 10% to 11 groups on any system (unless I am missing
> something). The sum of guarantees cannot exceed 100%.

That's exactly my point. I was trying to counter Bharata's statement, which was:

> > Now if 11th group with same shares comes in, then each group will now
> > get 9% of CPU and that 10% guarantee breaks.

which seemed to be implying that this was a drawback of using shares
to implement guarantees.

Paul
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