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    SubjectRe: [PATCH] async poll for 2.5
    Davide Libenzi wrote:
    >
    > On Tue, 15 Oct 2002, Benjamin LaHaise wrote:
    >
    > > On Tue, Oct 15, 2002 at 12:16:39PM -0700, Davide Libenzi wrote:
    > > > Ben, one of the reasons of the /dev/epoll speed is how it returns events
    > > > and how it collapses them. A memory mapped array is divided by two and
    > > > while the user consumes events in one set, the kernel fill the other one.
    > > > The next wait() will switch the pointers. There is no copy from kernel to
    > > > user space. Doing :
    > > >
    > > > int sys_epoll_wait(int epd, struct pollfd **pevts, int timeout);
    > > >
    > > > the only data the kernel has to copy to userspace is the 4(8) bytes for
    > > > the "pevts" pointer.
    > >
    > > Erm, the aio interface has support for the event ringbuffer being accessed
    > > by userspace (it lives in user memory and the kernel acts as a writer, with
    > > userspace as a reader), that's one of its advantages -- completion events
    > > are directly accessible from userspace after being written to by an
    > > interrupt. Ideally this is to be wrapped in a vsyscall, but we don't have
    > > support for that yet on x86, although much of the code written for x86-64
    > > should be reusable.
    >
    > In general I would like to have a "common" interface to retrieve IO
    > events, but IMHO the two solutions should be benchmarked before adopting
    > the one or the other.

    Seems like /dev/epoll uses a double-buffering scheme rather than
    a ring buffer, and this is not just a trivial difference; it's
    related to how redundant events are collapsed, right?
    - Dan
    -
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