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    SubjectRe: async network I/O, event channels, etc
    On Tue, Jul 25, 2006 at 03:01:22PM -0700, David Miller (davem@davemloft.net) wrote:
    > From: Ulrich Drepper <drepper@redhat.com>
    > Date: Tue, 25 Jul 2006 12:23:53 -0700
    >
    > > I was very much surprised by the reactions I got after my OLS talk.
    > > Lots of people declared interest and even agreed with the approach and
    > > asked me to do further ahead with all this. For those who missed it,
    > > the paper and the slides are available on my home page:
    > >
    > > http://people.redhat.com/drepper/
    > >
    > > As for the next steps I see a number of possible ways. The discussions
    > > can be held on the usual mailing lists (i.e., lkml and netdev) but due
    > > to the raw nature of the current proposal I would imagine that would be
    > > mainly perceived as noise.
    >
    > Since I gave a big thumbs up for Evgivny's kevent work yesterday
    > on linux-kernel, you might want to start by comparing your work
    > to his. Because his has the advantage that 1) we have code now
    > and 2) he has written many test applications and performed many
    > benchmarks against his code which has flushed out most of the
    > major implementation issues.
    >
    > I think most of the people who have encouraged your work are unaware
    > of Evgivny's kevent stuff, which is extremely unfortunate, the two
    > works are more similar than they are different.
    >
    > I do not think discussing all of this on netdev would be perceived
    > as noise. :)

    Hello David, Ulrich.

    Here is brief description of what is kevent and how it works.

    Kevent subsystem incorporates several AIO/kqueue design notes and ideas.
    Kevent can be used both for edge and level notifications. It supports
    socket notifications (accept, send, recv), network AIO (aio_send(),
    aio_recv() and aio_sendfile()), inode notifications (create/remove),
    generic poll()/select() notifications and timer notifications.

    There are several object in the kevent system:
    storage - each source of events (socket, inode, timer, aio, anything) has
    structure kevent_storage incorporated into it, which is basically a list
    of registered interests for this source of events.
    user - it is abstraction which holds all requested kevents. It is
    similar to FreeBSD's kqueue.
    kevent - set of interests for given source of events or storage.

    When kevent is queued into storage, it will live there until removed by
    kevent_dequeue(). When some activity is noticed in given storage, it
    scans it's kevent_storage->list for kevents which match activity event.
    If kevents are found and they are not already in the
    kevent_user->ready_list, they will be added there at the end.

    ioctl(WAIT) (or appropriate syscall) will wait until either requested
    number of kevents are ready or timeout elapsed or at least one kevent is
    ready, it's behaviour depends on parameters.

    It is possible to have one-shot kevents, which are automatically removed
    when are ready.

    Any event can be added/removed/modified by ioctl or special controlling
    syscall.

    Network AIO is based on kevent and works as usual kevent storage on top
    of inode.
    When new socket is created it is associated with that inode and when
    some activity is detected appropriate notifications are generated and
    kevent_naio_callback() is called.
    When new kevent is being registered, network AIO ->enqueue() callback
    simply marks itself like usual socket event watcher. It also locks
    physical userspace pages in memory and stores appropriate pointers in
    private kevent structure. I have not created additional DMA memory
    allocation methods, like Ulrich described in his article, so I handle it
    inside NAIO which has some overhead (I posted get_user_pages()
    sclability graph some time ago).
    Network AIO callback gets pointers to userspace pages and tries to copy
    data from receiving skb queue into them using protocol specific
    callback. This callback is very similar to ->recvmsg(), so they could
    share a lot in future (as far as I recall it worked only with hardware
    capable to do checksumming, I'm a bit lazy).

    Both network and aio implementation work on top of hooks inside
    appropriate state machines, but not as repeated call design (curect AIO)
    or special thread (SGI AIO). AIO work was stopped, since I was unable to
    achieve the same speed as synchronous read
    (maximum speeds were 2Gb/sec vs. 2.1 GB/sec for aio and sync IO accordingly
    when reading data from the cache).
    Network aio_sendfile() works lazily - it asynchronously populates pages
    into the VFS cache (which can be used for various tricks with adaptive
    readahead) and then uses usual ->sendfile() callback.

    I have not created an interface for userspace events (like Solaris),
    since right now I do not see it's usefullness, but if there is
    requirements for that it is quite easy with kevents.

    I'm preparing set of kevent patches resend (with cleanups mentioned in
    previous e-mails), which will be ready in a couple of moments.

    1. kevent homepage.
    http://tservice.net.ru/~s0mbre/old/?section=projects&item=kevent

    2. network aio homepage.
    http://tservice.net.ru/~s0mbre/old/?section=projects&item=naio

    3. LWN.net published a very good article about kevent.
    http://lwn.net/Articles/172844/

    Thank you.

    --
    Evgeniy Polyakov
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