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SubjectRe: [RFC] CPU hard limits
* menage@google.com <menage@google.com> [2009-06-05 01:53:15]:

> 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?
>
> 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.
>
> Suppose cgroup A wants a guarantee of 50% and two others, B and C,
> want guarantees of 15% each; give A 50 shares and B and C 15 shares
> each. In this case, if they all run flat out they'll get 62%/19%/19%,
> which is within their SLA.
>
> That's not to say that hard limits can't be useful in their own right
> - e.g. for providing reproducible loadtesting conditions by
> controlling how much CPU a service can use during the load test. But I
> don't see why using them to implement guarantees is either necessary
> or desirable.
>
> (Unless I'm missing some crucial point ...)

The important scenario I have is adding and removing groups.

Consider 10 cgroups with shares of 10 each, what if 5 new are created
with the same shares? We now start getting 100/15, even though we did
not change our shares.

--
Balbir


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