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

[Andrea Arcangeli]

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.

> 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