Re: [RFC] CPU hard limits

Bharata B Rao
2009-06-05T09:28:41Z

On Fri, Jun 05, 2009 at 01:53:15AM -0700, Paul Menage wrote:
> On Wed, Jun 3, 2009 at 10:36 PM, Bharata B
> Rao<bharata@linux.vnet.ibm.com> wrote:
> > - Hard limits can be used to provide guarantees.
> >
>
> This claim (and the subsequent long thread it generated on how limits
> can provide guarantees) confused me a bit.
>
> Why do we need limits to provide guarantees when we can already
> provide guarantees via shares?

shares design is proportional and hence it can't by itself provide
guarantees.

>
> 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.

Regards,
Bharata.