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    SubjectRe: [RFC PATCH 0/4] Gang scheduling in CFS
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    On Mon, 2011-12-19 at 16:51 +0100, Peter Zijlstra wrote:
    > 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?

    No we can run separate mm contexts, but we can only run one -partition-
    at a time. Sadly the host kernel is also a partition for the MMU so that
    means that all 4 threads must be running the same guest and enter/exit
    the guest at the same time.

    > 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.

    Cheers,
    Ben.





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