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    SubjectRe: [RFC] CPU hard limits
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    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


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