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    SubjectRe: [PATCH v4 0/5] getcpu_cache system call for 4.6
    ----- On Feb 23, 2016, at 8:36 PM, H. Peter Anvin hpa@zytor.com wrote:

    > On 02/23/2016 03:28 PM, Mathieu Desnoyers wrote:
    >> Hi,
    >>
    >> Here is a patchset implementing a cache for the CPU number of the
    >> currently running thread in user-space.
    >>
    >> Benchmarks comparing this approach to a getcpu based on system call on
    >> ARM show a 44x speedup. They show a 14x speedup on x86-64 compared to
    >> executing lsl from a vDSO through glibc.
    >>
    >> I'm added a man page in the changelog of patch 1/3, which shows an
    >> example usage of this new system call.
    >>
    >> This series is based on v4.5-rc5, submitted for Linux 4.6.
    >>
    >> Feedback is welcome,
    >>
    >
    > What is the resulting context switch overhead?

    The getcpu_cache only adds code to the thread migration path,
    and to the resume notifier. The context switch path per se is
    untouched. I would therefore expect the overhead on context
    switch to be within the noise, except if stuff like hackbench
    would be so sensitive to the size of struct task_struct that
    a single extra pointer added at the end of struct task_struct
    would throw off the benchmarks.

    Is that what you are concerned about ?

    Thanks,

    Mathieu

    --
    Mathieu Desnoyers
    EfficiOS Inc.
    http://www.efficios.com

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