Re: [PATCH] io stalls

From: Nick Piggin (
Date: Wed Jun 11 2003 - 20:04:42 EST

Andrea Arcangeli wrote:

>On Wed, Jun 11, 2003 at 02:27:13PM -0400, Chris Mason wrote:
>>On Wed, 2003-06-11 at 14:12, Andrea Arcangeli wrote:
>>>On Wed, Jun 11, 2003 at 01:42:41PM -0400, Chris Mason wrote:
>>>>+ if (q->rq[rw].count >= q->batch_requests) {
>>>>+ smp_mb();
>>>>+ if (waitqueue_active(&q->wait_for_requests[rw]))
>>>>+ wake_up(&q->wait_for_requests[rw]);
>>>in my tree I also changed this to:
>>> wake_up_nr(&q->wait_for_requests[rw], q->rq[rw].count);
>>>otherwise only one waiter will eat the requests, while multiple waiters
>>>can eat requests in parallel instead because we freed not just 1 request
>>>but many of them.
>>I tried a few variations of this yesterday and they all led to horrible
>>latencies, but I couldn't really explain why. I had a bunch of other
>the I/O latency in theory shouldn't change, we're not reordering the
>queue at all, they'll go to sleep immediatly again if __get_request
>returns null.

And go to the end of the queue?

>>stuff in at the time to try and improve throughput though, so I'll try
>>it again.
>>I think part of the problem is the cascading wakeups from
>>get_request_wait_wakeup(). So if we wakeup 32 procs they in turn wakeup
>>another 32, etc.
>so maybe it's enough to wakeup count / 2 to account for the double
>wakeup? that will avoid some overscheduling indeed.

Andrea, this isn't needed because when the queue falls below
the batch limit, every retiring request will do a wake up and
another request will be put on (as long as the waitqueue is

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