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    SubjectRe: [PATCH 07/32] cpuset: Set up interface for nohz flag
    On Wed, Mar 21, 2012 at 09:50:27AM -0500, Christoph Lameter wrote:
    > On Wed, 21 Mar 2012, Frederic Weisbecker wrote:
    >
    > > Prepare the interface to implement the nohz cpuset flag.
    > > This flag, once set, will tell the system to try to
    > > shutdown the periodic timer tick when possible.
    > >
    > > We use here a per cpu refcounter. As long as a CPU
    > > is contained into at least one cpuset that has the
    > > nohz flag set, it is part of the set of CPUs that
    > > run into adaptive nohz mode.
    >
    > What are the drawbacks for nohz?
    >
    > If there are none: Can we make nohz default behavior without relying on
    > cpusets?

    I can't tell for now. I haven't yet covered everything the timer is
    handling. Until that happens I can't do measurements.

    In theory this sounds like a win in every case. I just would like
    to test that in practice. This sets up hooks in kernel entry/exit.
    More IPIs here and there. May be this adds overhead on workloads
    involving a lot of syscalls or exceptions. I don't know.

    Given this is not yet entirely clear, I think it may be better to keep
    this interface around until the patchset reaches a version that becomes
    mergeable. Then at this point we can get serious testing coverage to take
    the decision to drop the interface and make it unconditional on
    CONFIG_ADAPTIVE_NO_HZ.


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