[Devel] Re: [RFC] CPU hard limits

[Paul Menage]

On Fri, Jun 5, 2009 at 2:27 AM, Bharata B Rao<bharata at linux.vnet.ibm.com> wrote:
>>
>> Suppose 10 cgroups each want 10% of the machine's CPU. We can just
>> give each cgroup an equal share, and they're guaranteed 10% if they
>> try to use it; if they don't use it, other cgroups can get access to
>> the idle cycles.
>
> 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 al 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?

Paul
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