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    SubjectRe: [PATCH] x86: Run checksumming in parallel accross multiple alu's

    * Neil Horman <nhorman@tuxdriver.com> wrote:

    > On Sat, Oct 12, 2013 at 07:21:24PM +0200, Ingo Molnar wrote:
    > >
    > > * Neil Horman <nhorman@tuxdriver.com> wrote:
    > >
    > > > Sébastien Dugué reported to me that devices implementing ipoib (which
    > > > don't have checksum offload hardware were spending a significant amount
    > > > of time computing checksums. We found that by splitting the checksum
    > > > computation into two separate streams, each skipping successive elements
    > > > of the buffer being summed, we could parallelize the checksum operation
    > > > accros multiple alus. Since neither chain is dependent on the result of
    > > > the other, we get a speedup in execution (on hardware that has multiple
    > > > alu's available, which is almost ubiquitous on x86), and only a
    > > > negligible decrease on hardware that has only a single alu (an extra
    > > > addition is introduced). Since addition in commutative, the result is
    > > > the same, only faster
    > >
    > > This patch should really come with measurement numbers: what performance
    > > increase (and drop) did you get on what CPUs.
    > >
    > > Thanks,
    > >
    > > Ingo
    > >
    >
    >
    > So, early testing results today. I wrote a test module that, allocated
    > a 4k buffer, initalized it with random data, and called csum_partial on
    > it 100000 times, recording the time at the start and end of that loop.

    It would be nice to stick that testcase into tools/perf/bench/, see how we
    are able to benchmark the kernel's mempcy and memset implementation there:

    $ perf bench mem memcpy -r help
    # Running 'mem/memcpy' benchmark:
    Unknown routine:help
    Available routines...
    default ... Default memcpy() provided by glibc
    x86-64-unrolled ... unrolled memcpy() in arch/x86/lib/memcpy_64.S
    x86-64-movsq ... movsq-based memcpy() in arch/x86/lib/memcpy_64.S
    x86-64-movsb ... movsb-based memcpy() in arch/x86/lib/memcpy_64.S

    In a similar fashion we could build the csum_partial() code as well and do
    measurements. (We could change arch/x86/ code as well to make such
    embedding/including easier, as long as it does not change performance.)

    Thanks,

    Ingo
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