Re: [RFC] CPU hard limits

Dhaval Giani
2009-06-05T09:48:32Z

On Fri, Jun 05, 2009 at 02:32:51AM -0700, Paul Menage wrote:
> On Fri, Jun 5, 2009 at 2:27 AM, Bharata B Rao<bharata@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?
>

You cannot guarantee 10% to 11 groups on any system (unless I am missing
something). The sum of guarantees cannot exceed 100%.

How would you be able to do that with any other mechanism?

Thanks,
--
regards,
Dhaval