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    SubjectRe: [RFC] CPU hard limits
    On Fri, Jun 05, 2009 at 04:02:11PM +0300, Avi Kivity wrote:
    > Paul Menage wrote:
    >> On Wed, Jun 3, 2009 at 10:36 PM, Bharata B
    >> Rao<> 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 ...)
    > How many shares does a cgroup with a 0% guarantee get?

    Shares cannot be used to provide guarantees. All they decide is what
    propotion groups can get CPU time. (yes, shares is a bad name, weight
    shows the intent better).


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