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    SubjectRe: [PATCH] io stalls


    Nick Piggin wrote:

    >
    >
    > 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.


    I guess you could fix this by having a "last woken" flag, and
    allow that process to allocate requests without blocking from
    the batch limit until the queue full limit. That is how
    batch_requests is supposed to work.

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