Re: [PATCH] io stalls

[Nick Piggin]

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.