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    SubjectRe: [PATCH] dax, pmem: add support for msync
    On 09/02/2015 05:23 PM, Dave Hansen wrote:
    <>
    > I'd be curious what the cost is in practice. Do you have any actual
    > numbers of the cost of doing it this way?
    >
    > Even if the instruction is a "noop", I'd really expect the overhead to
    > really add up for a tens-of-gigabytes mapping, no matter how much the
    > CPU optimizes it.

    What tens-of-gigabytes mapping? I have yet to encounter an application
    that does that. Our tests show that usually the mmaps are small.

    I can send you a micro benchmark results of an mmap vs direct-io random
    write. Our code will jump over holes in the file BTW, but I'll ask to also
    run it with falloc that will make all blocks allocated.

    Give me a few days to collect this.

    I guess one optimization we should do is jump over holes and zero-extents.
    This will save the case of a mostly sparse very big file.

    Thanks
    Boaz



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