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    SubjectRe: [RFC] CPU hard limits
    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?
    >
    > 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?

    Ideally, the scheduler would hand out cpu time according to weight and
    demand, then clamp over-demand by a cgroup's limit and boost the share
    to meet guarantees.

    --
    I have a truly marvellous patch that fixes the bug which this
    signature is too narrow to contain.



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