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    SubjectRe: [PATCH] io stalls


    Andrea Arcangeli wrote:

    >On Thu, Jun 12, 2003 at 12:41:58PM +1000, Nick Piggin wrote:
    >
    >>
    >>Chris Mason wrote:
    >>
    >>
    >>>On Wed, 2003-06-11 at 21:29, Andrea Arcangeli wrote:
    >>>
    >>>
    >>>
    >>>>this will avoid get_request_wait_wakeup to mess the wakeup, so we can
    >>>>wakep_nr(rq.count) safely.
    >>>>
    >>>>then there's the last issue raised by Chris, that is if we get request
    >>>>released faster than the tasks can run, still we can generate a not
    >>>>perfect fairness. My solution to that is to change wake_up to have a
    >>>>nr_exclusive not obeying to the try_to_wakeup retval. that should
    >>>>guarantee exact FIFO then, but it's a minor issue because the requests
    >>>>shouldn't be released systematically in a flood. So I'm leaving it
    >>>>opened for now, the others already addressed should be the major ones.
    >>>>
    >>>>
    >>>I think the only time we really need to wakeup more than one waiter is
    >>>when we hit the q->batch_request mark. After that, each new request
    >>>that is freed can be matched with a single waiter, and we know that any
    >>>previously finished requests have probably already been matched to their
    >>>own waiter.
    >>>
    >>>
    >>>
    >>Nope. Not even then. Each retiring request should submit
    >>a wake up, and the process will submit another request.
    >>So the number of requests will be held at the batch_request
    >>mark until no more waiters.
    >>
    >>Now that begs the question, why have batch_requests anymore?
    >>It no longer does anything.
    >>
    >
    >it does nothing w/ _exclusive and w/o the wake_up_nr, that's why I added
    >the wake_up_nr.
    >
    >
    That is pretty pointless as well. You might as well just start
    waking up at the queue full limit, and wake one at a time.

    The purpose for batch_requests was I think for devices with a
    very small request size, to reduce context switches.

    >Andrea
    >
    >
    >

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