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    SubjectRe: [PATCH 2/4] ftrace - add function_duration tracer
    On Thu, 2009-12-10 at 14:36 -0800, Tim Bird wrote:

    > The ring buffer code was particularly slow. I was using
    > a simple fixed-size array in KFT (on a uniprocessor), and
    > the time to save an event was under 1 microsecond. With
    > ftrace on the same hardware, it costs about 6 microseconds
    > to save an event.

    Well, I know you work in embedded so it may be much slower.

    I just ran the ring_buffer_benchmark on a Xeon Quad 2660MHz with
    2.6.32-rc7 and I got 190ns per trace entry. (Note most of this time is
    spent in the timestamp calculation!)

    I booted the latest 2.6.32 kernel, where I've made some changes to the
    time keeping code and got 117ns per entry. I sent out a RFC patch set,
    and disabled all the RB_WARN_ON safety checks and got it down to 90ns
    per trace entry! This still includes the 3 levels of checking if the
    buffer is enabled. Those checks are just a nit and probably (haven't
    tested) cost a single ns. The variables being checked are labeled
    read_mostly and are most likely in the cache.

    Maybe you want to test out the overhead again?

    Note, the ftrace infrastructure's first priority is stability so I won't
    be removing those RB_WARN_ONs. Next priority is to have non tracing be
    unaffected. Third is tracing being as fast as possible. A tracer that
    does not need to worry about going away when disabled will probably
    have a performance advantage when tracing. But, if it causes a
    noticeable overhead when disabled (like -finstrument-function does, and
    it has a lot more lines than mcount), it will never be merged.

    -- Steve

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