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    SubjectRe: [RFC] sched: implement the exclusive wait queue as a LIFO queue
    On Wed, Apr 28, 2010 at 9:21 PM, Jamie Lokier <jamie@shareable.org> wrote:
    > Changli Gao wrote:
    >>
    >> fs/eventpoll.c: 1443.
    >>                 wait.flags |= WQ_FLAG_EXCLUSIVE;
    >>                 __add_wait_queue(&ep->wq, &wait);
    >
    > The same thing about assumptions applies here.  The userspace process
    > may be waiting for an epoll condition to get access to a resource,
    > rather than being a worker thread interchangeable with others.

    Oh, the lines above are the current ones. So the assumptions applies
    and works here.

    >
    > For example, userspace might be using a pipe as a signal-safe lock, or
    > signal-safe multi-token semaphore, and epoll to wait for that pipe.
    >
    > WQ_FLAG_EXCLUSIVE means there is no point waking all tasks, to avoid a
    > pointless thundering herd.  It doesn't mean unfairness is ok.

    The users should not make any assumption about the waking up sequence,
    neither LIFO nor FIFO.

    >
    > The LIFO idea _might_ make sense for interchangeable worker-thread
    > situations - including userspace.  It would make sense for pipe
    > waiters, socket waiters (especially accept), etc.

    Yea, and my following patches are for socket waiters.

    >
    > Do you have any measurements which showing the LIFO mode performing
    > better than FIFO, and by how much?
    >

    I didn't do any test yet. But some work done by LSE project years ago
    showed that it is better.

    http://lse.sourceforge.net/io/aionotes.txt

    " Also in view of
    better cache utilization the wake queue mechanism is LIFO by default.
    (A new exclusive LIFO wakeup option has been introduced for this purpose)"

    --
    Regards,
    Changli Gao(xiaosuo@gmail.com)
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