Re: [PATCH] io stalls

Nick Piggin
2003-06-12T02:36:28Z

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
>
>
>