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    SubjectRe: [patch] futex API cleanups, futex-api-cleanup-2.5.69-A2
    Davide Libenzi wrote:
    > > Yes, they do and it should work (I haven't tried, though).
    > >
    > > There is a practical problem when waiting on a futex in multiple
    > > threads using epoll: you need a separate fd per waiter, rather than an
    > > fd per waited-on futex. This is because some uses of futexes depend
    > > on waiters being woken in the exact order that they were queued.
    > Not really Jamie. See a Futex event is not much different from a network
    > one. When the event shows up, one thread will pick it up (epoll_wait) and
    > will handle it. A futex event will very likely be a green light for some
    > sort of resource usage that whatever thread can pick up and handle.

    That is true when a futex represents some item of work to do, or
    readiness such as data coming in or going it. Then it is quite
    reasonable to think of it like a pollable fd.

    But futexes are also used to represent contention for shared
    resources, and the properties needed for this are quite different.

    See futex_up_fair() in Rusty's futex-2.2 library. That depends
    crucially on getting exactly the right number of wakeup tokens passed,
    and in the order the waiters blocked.

    To pick an example, consider a dynamic web server which is
    occasionally asked to render complex images, but where most of the
    content is easily generated. You might have 30 concurrent page
    serving threads, but want to limit the number of threads which are
    generating a particularly complex response to 3 at a time because of
    memory constraints.

    You cannot program this as putting things on a work queue and having
    arbitrary threads pick them off, unless you are prepared to represent
    the problem as an explicit state machine, where the intermediate
    states can be represented as a data structure. If the state is too
    complex for that, which is often a reason for using threads in the
    first place, that is not an option.

    So it is important that _those_ kind of waiting threads are woken in
    something approximating the order they went to sleep. Otherwise there
    is a danger of certain requests being starved unfairly.

    To implement this you can use a simple futex, or a significantly more
    complex, and slower, queue structure.

    Using the futex, you either need an fd per waiter to get this
    fairness, when the waiters are using epoll to listen for more than one
    event, _or_ you need epoll to be clever and preserve the sleep-to-wake
    ordering over a single futex. I favour that last solution.

    -- Jamie

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