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    SubjectRe: [PLEASE-TESTME] Zerocopy networking patch, 2.4.0-1
    In article <>,
    Stephen C. Tweedie <> wrote:
    >Jes has also got hard numbers for the performance advantages of
    >jumbograms on some of the networks he's been using, and you ain't
    >going to get udp jumbograms through a page-by-page API, ever.


    The only thing you need is a nagle-type thing that coalesces requests.
    In the case of UDP, that coalescing obviously has to be explicitly
    controlled, as the "standard" UDP behaviour is to send out just one
    packet per write.

    But this is a problem for TCP too: you want to tell TCP to _not_ send
    out a short packet even if there are none in-flight, if you know you
    want to send more. So you want to have some way to anti-nagle for TCP

    Also, if you look at the problem of "writev()", you'll notice that you
    have many of the same issues: what you really want is to _always_
    coalesce, and only send out when explicitly asked for (and then that
    explicit ask would be on by default at the end of write() and at the
    very end of the last segment in "writev()".

    It so happens that this logic already exists, it's called MSG_MORE or
    something similar (I'm too lazy to check the actual patches).

    And it's there exactly because it is stupid to make the upper layers
    have to gather everything into one packet if the lower layers need that
    logic for other reasons anyway. Which they obviously do.

    So what you can do is to just do multiple writes, and set the MSG_MORE
    flag. This works with sendfile(), but more importantly it is also an
    uncommonly good interface to user mode. With this, you can actually
    implement things like "writev()" _properly_ from user-space, and we
    could get rid of the special socket writev() magic if we wanted to.

    So if you have a header, you just send out that header separately (with
    the MSG_MORE flag), and then do a "sendfile()" or whatever to send out
    the data.

    This is much more flexible than writev(), and a lot easier to use. It's
    also a hell of a lot more flexible than the ugly sendfile() interfaces
    that HP-UX and the BSD people have - I'm ashamed of how little taste the
    BSD group in general has had in interface design. Ugh. Tacking on a
    mixture of writev() and sendfile() in the same system call. Tacky.

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