Re: [PATCH] rq-dyn-alloc, dynamic request allocation

[Jens Axboe <axboe@suse.de>]

On Tue, Apr 01 2003, Andrew Morton wrote:
> Jens Axboe <axboe@suse.de> wrote:
> >
> > Hi,
> > 
> > This patch adds dynamic request allocation to the block io path. On
> > systems with lots of disks (and thus queues) it saves a non-significant
> > amount of low memory. It also allows for much better experimentation
> > with larger queue lengths, this experimental patch tops the queue depth
> > off at 16384 (vs 128 before).
> 
> heh, 16k requests per queue?  Last time I played with 1024 certain popular
> benchmarks ran like a bullet.

I have one such unmentionable benchmark running right now, current depth
is 5127 :-)

> > Please play with it. Andrew, want a version for -mm?
> 
> Would be much appreciated, thanks.

Ok, getting on it.

> >   */
> >  static struct request *get_request_wait(request_queue_t *q, int rw)
> >  {
> > -	DEFINE_WAIT(wait);
> > -	struct request_list *rl = &q->rq[rw];
> >  	struct request *rq;
> >  
> > -	spin_lock_prefetch(q->queue_lock);
> > -
> >  	generic_unplug_device(q);
> >  	do {
> > -		int block = 0;
> > +		rq = get_request(q, rw, GFP_NOIO);
> >  
> > -		prepare_to_wait_exclusive(&rl->wait, &wait,
> > -					TASK_UNINTERRUPTIBLE);
> > -		spin_lock_irq(q->queue_lock);
> > -		if (!rl->count)
> > -			block = 1;
> > -		spin_unlock_irq(q->queue_lock);
> > -
> > -		if (block)
> > +		if (!rq)
> >  			io_schedule();
> 
> hmm.  I fear that if a SCHED_FIFO/SCHED_RR task hits this, it will just pick
> itself to run again in the schedule() and the box locks up.
> 
> A blk_congestion_wait(WRITE, HZ/50) may be better here.  It will send the
> caller to sleep until someone puts a write request back, which seems
> appropriate.

Yes good point, I'll make that change.

-- 
Jens Axboe