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    SubjectRe: [RFC PATCH 2/2] mm/sparse: add last_section_nr in sparse_init() to reduce some iteration cycle
    On Mon, Mar 06, 2017 at 02:42:25PM -0500, Tejun Heo wrote:
    >Hello, Wei.
    >
    >On Fri, Feb 17, 2017 at 10:12:31PM +0800, Wei Yang wrote:
    >> > And compare the ruling with the iteration for the loop to be (1UL <<
    >> > 5) and (1UL << 19).
    >> > The runtime is 0.00s and 0.04s respectively. The absolute value is not much.
    >
    >systemd-analyze usually does a pretty good job of breaking down which
    >phase took how long. It might be worthwhile to test whether the
    >improvement is actually visible during the boot.
    >

    Hi, Tejun

    Thanks for your suggestion. I have tried systemd-analyze to measure the
    effect, while looks not good.

    Result without patch
    -------------------------
    Startup finished in 7.243s (kernel) + 25.034s (userspace) = 32.277s
    Startup finished in 7.254s (kernel) + 19.816s (userspace) = 27.071s
    Startup finished in 7.272s (kernel) + 4.363s (userspace) = 11.636s
    Startup finished in 7.258s (kernel) + 24.319s (userspace) = 31.577s
    Startup finished in 7.262s (kernel) + 9.481s (userspace) = 16.743s
    Startup finished in 7.266s (kernel) + 14.766s (userspace) = 22.032s

    Avg = 7.259s

    Result with patch
    -------------------------
    Startup finished in 7.262s (kernel) + 14.294s (userspace) = 21.557s
    Startup finished in 7.264s (kernel) + 19.519s (userspace) = 26.783s
    Startup finished in 7.266s (kernel) + 4.730s (userspace) = 11.997s
    Startup finished in 7.258s (kernel) + 9.514s (userspace) = 16.773s
    Startup finished in 7.258s (kernel) + 14.371s (userspace) = 21.629s
    Startup finished in 7.258s (kernel) + 14.627s (userspace) = 21.885s

    Avg = 7.261s

    It looks the effect is not obvious. Maybe the improvement is not good
    enough :(

    >> >> * Do we really need to add full reverse iterator to just get the
    >> >> highest section number?
    >> >>
    >> >
    >> > You are right. After I sent out the mail, I realized just highest pfn
    >> > is necessary.
    >
    >That said, getting efficient is always great as long as the added
    >complexity is justifiably small enough. If you can make the change
    >simple enough, it'd be a lot easier to merge.
    >

    Agree.

    I have replaced the reverse iteration with a simple last pfn return. The test
    result above is based on the new version.

    >Thanks.
    >
    >--
    >tejun

    --
    Wei Yang
    Help you, Help me
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