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    SubjectRe: [patch] Prefetching file_read_actor()
    Dave Jones wrote:
    > After noticing file_read_actor() showing up in profiles quite
    > a bit, I grepped old l-k messages, and turned up a post by
    > Manfred Spraul in which he posted a patch using inline asm
    > to prefetch read data. This was x86 specific in generic code,
    > so was a little hackish..
    > Now that we have the prefetch macros, I decided to play with
    > this a little tonight, and came up with this patch..

    Well I don't know diddly about the prefetch instruction, but.

    > ...
    > +
    > + if (size > 128) {
    > + int i;
    > + for(i=0; i<size; i+=64) {
    > + prefetch (kaddr+offset);
    > + prefetch (kaddr+offset+(L1_CACHE_BYTES*2));
    > + }
    > + }
    > +
    > left = __copy_to_user(desc->buf, kaddr + offset, size);
    > kunmap(page);

    This needs to be inside ARCH_HAS_PREFETCH. Otherwise, for
    architectures which do not implement prefetch, we have

    for (i = 0; i < size; i += 64) {

    and the compiler will *not* optimise this away. The compiler deliberately
    leaves this construct alone because it assumes the programmer is asking for
    a busywait loop.

    We shouldn't add a busywait loop to file_read_actor(), yes?

    Reminds me of a version of the Microsoft C compiler back in about '85 which
    "optimise" away

    for ( ; ; )


    Is prefetching 4k at a time optimal? Should the prefetch be embedded
    in copy_*_user?

    The code assumes that L1_CACHE_BYTES equates to the prefetch chunk.
    Is this reasonable, or should it be abstracted out to a new PREFETCH_BYTES?

    That `64' needs to be PREFETCH_BYTES * 2 or L1_CACHE_BYTES * 2, yes?

    How come the code keeps prefetching the same address? Shouldn't
    we be adding `i' to the address in there?

    The code makes no attempt to align the prefetch address to anything.
    Should it?

    What happens if a prefetch spills over the end of the page and

    > Comments ?

    That'll do for now :)

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