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    SubjectRe: [PATCH 2.6.21 review I] [21/25] x86_64: a memcpy that tries to reduce cache pressure
    Andi Kleen a écrit :
    > From: "Bryan O'Sullivan" <bos@pathscale.com>
    >
    > This copy routine is memcpy-compatible, but on some architectures will use
    > cache-bypassing loads to avoid bringing the source data into the cache.
    >
    > One case where this is useful is when a device issues a DMA to a memory
    > region, and the CPU must copy the DMAed data elsewhere before doing any work
    > with it. Since the source data is read-once, write-never from the CPU's
    > perspective, caching the data at those addresses can only evict potentially
    > useful data.
    >
    > We provide an x86_64 implementation that uses SSE non-temporal loads, and a
    > generic version that falls back to plain memcpy.

    > + movq %r11, 56(%rdi)
    > + addq %rcx, %rdi
    > + cmpq %rdx, %rcx /* is rdx >= 64? */
    > + jbe .L42
    > + sfence
    > + orl %edx, %edx
    > + je .L33

    I have three questions/remarks

    1) Just curious why sfence is necessary here ?

    2) Shouldnt we use this for large buffers, and restrict them to a size
    multiple of 64, to avoid all these conditional branches ?

    3) Also, the first 128 bytes of the source buffer will be bring into cache.

    -
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