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    SubjectRe: [RFC PATCH 0/4] Gang scheduling in CFS
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    On Mon, 2011-12-19 at 14:03 +0530, Nikunj A. Dadhania wrote:
    > The following patches implements gang scheduling. These patches
    > are *highly* experimental in nature and are not proposed for
    > inclusion at this time.

    Nor will they ever be, I've always strongly opposed the whole concept
    and I'm not about to change my mind. Gang scheduling is a scalability
    nightmare.

    > Gang scheduling can be helpful in virtualization scenario. It will
    > help in avoiding the lock-holder-preemption[1] problem and other
    > benefits include improved lock-acquisition times. This feature
    > will help address some limitations of KVM on Power

    Use paravirt ticket locks or a pause-loop-filter like thing.

    > On Power, we have an interesting hardware restriction on guests
    > running across SMT theads: on any single core, we can only run one
    > mm context at any given time.

    OMFG are your hardware engineers insane?

    Anyway, I had a look at your patches and I don't see how could ever
    work. You gang-schedule cgroup entities, but there's no guarantee the
    load-balancer will have at least one task for each group on every cpu.




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