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    SubjectRe: PATCH: Raw device IO for 2.1.131


    On Sun, 13 Dec 1998, Richard Gooch wrote:
    >
    > I think Linus' point may be that the extra time spent copying data
    > around is not that significant.

    Exactly. It _is_ significant if the _only_ thing you do is copy the data
    around, and that's why I do like sendfile() as a concept: the concept of
    sendfile() is that a pure "copy" operation is very common.

    > What I'd like to know is why the transfer to user space memory is
    > deemed so performance critical. Most likely, if you are going to *any*
    > processing of that data, the CPU and memory IO requirements will be
    > far greater than the kernel->userspace copy. Is that extra copy really
    > going to matter? As Linus said, maybe it's time to buy a new computer
    > if you're running that close to the wire.

    Yes. I'm all for making old computers work well with Linux too, but at the
    same time I see all new PC class chips coming out with memory interfaces
    in the multiple GB/s range, and with 1MB+ on-chip cache. No, it may not be
    common today, but it _is_ going to be common in just a few years.

    And yes, you'll have PCI++ or IONG or whatever, but I think that basically
    the computer manufacturers woke up to memory speeds, and I don't expect IO
    to catch up. Memory is basically just always going to be faster than IO
    unless the memory subsystem is really bad - memory _is_ IO, but of the
    simplest and fastest kind.

    Linus' prediction: memory will be at least an order of magnitude faster
    than the IO bandwidth on machines that real people use. And I worry more
    about cleanliness than about zero-copy in many cases.

    > Linus: have I correctly interpreted your point?

    Pretty much.

    Linus


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