Re: [PATCH] io stalls (was: -rc7 Re: Linux 2.4.21-rc6)

[Chris Mason <mason@suse.com>]

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
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.

-chris