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    SubjectRe: [PATCH RFC] [X86] performance improvement for memcpy_64.S by fast string.

    * H. Peter Anvin <hpa@zytor.com> wrote:

    > On 11/12/2009 11:33 PM, Ingo Molnar wrote:
    > >
    > > * Pavel Machek <pavel@ucw.cz> wrote:
    > >
    > >>> Ling, if you are interested, could you send a user-space test-app to
    > >>> this thread that everyone could just compile and run on various older
    > >>> boxes, to gather a performance profile of hand-coded versus string ops
    > >>> performance?
    > >>>
    > >>> ( And i think we can make a judgement based on cache-hot performance
    > >>> alone - if then the strings ops will perform comparatively better in
    > >>> cache-cold scenarios, so the cache-hot numbers would be a conservative
    > >>> estimate. )
    > >>
    > >> Ugh, really? I'd expect cache-cold performance to be not helped at all
    > >> (memory bandwidth limit) and you'll get slow down from additional
    > >> i-cache misses...
    > >
    > > That's my point - the new code is shorter, which will run comparatively
    > > faster in a cache-cold environment.
    > >
    >
    > memcpy_c by itself is by far the shortest variant, of course.

    yep. The argument i made was when a long function was compared to a
    short one. As you noted we dont actually enable the long function all
    that often - which inverts the same argument.

    > The question is if it makes sense to use the long variants for short
    > (< 1024 bytes) copies.

    I'd say not - the kernel executes in a icache-cold environment most of
    the time (as user-space is far more cache intense in the majority of
    workloads and kernel processing starts with a cold icache), so
    optimizing the kernel for code size is very important. (but numbers done
    on real workloads can convince me of the opposite.)

    Ingo


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