Messages in this thread | | | Date | Fri, 5 Jun 2009 01:53:15 -0700 | Subject | Re: [RFC] CPU hard limits | From | Paul Menage <> |
| |
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 ...)
Paul
| |