[RFC] CPU hard limits

[Paul Menage]

On Wed, Jun 3, 2009 at 10:36 PM, Bharata B
Rao<bharata at 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 ...)

Paul