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    SubjectRe: zero-copy TCP
    On Sat, Sep 02, 2000 at 04:28:18PM -0600, Jeff V. Merkey wrote:
    >
    >
    > Alan Cox wrote:
    > >
    > > We dont copy for checksumming. We fold the single user space copy and the
    > > checksum operation into one path, because on any modern CPU it costs precisely
    > > the same to copy as to copy/checksum.
    >
    > You stated in an earlier message you copied the data when you caclulated
    > the TCPIP checksum? No you say you don't. Perhaps I misunderstood.

    Linux always does a single copy for TCP, and the checksum is folded into
    that. Doing just the checksum alone wouldn't be much less costly.

    [Note this is only true for 2.4 in the fast path, 2.2 RX usually does
    checksum and copy-to-user separated, unless you have hardware RX checksumming

    For TX we always do a single copy checksum out of user space or out of the
    page cache when you use sendfile or mmap]


    -Andi
    -
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