Re: [RFC PATCH 2.6.9-rc2] Add sysfs queue depth override to qla2xxx

[Jeremy Higdon <jeremy@sgi.com>]

On Wed, Sep 29, 2004 at 08:21:09AM +0200, Jens Axboe wrote:
> On Tue, Sep 28 2004, Andrew Vasquez wrote:
> > On Tue, 28 Sep 2004, Jens Axboe wrote:
> > > On Tue, Sep 28 2004, James Bottomley wrote:
> > > > 
> > > > Well, don't go overboard on this ... there is a limit to the number of
> > > > outstanding requests any queue can have at one time...there's not much
> > > > point going over that since the block layer will throttle you when you
> > > > reach it.  I think it's 128, but it might be 256 ... but anyway, not
> > > > much larger.
> > > 
> > > That depends on both the io scheduler and the nr_requests setting, I
> > > don't think you should rely on any imposed block layer limit.
> > > 
> > 
> > So using scsi_device->request_queue->nr_requests as a highwater mark
> > is out of the question?  Looking through the call-chain during
> 
> It can change anytime.
> 
> > queue-depth size manipulation:
> > 
> > 	scsi_adjust_queue()
> > 	  blk_queue_resize_tags()
> > 	    init_tag_map()
> > 
> > there appears to be a restriction of (rq->nr_requests * 2) entries:
> > 
> > 	if (depth > q->nr_requests * 2) {
> > 		depth = q->nr_requests * 2;
> > 		printk(KERN_ERR "%s: adjusted depth to %d\n",
> > 				__FUNCTION__, depth);
> > 	}
> > 
> > What's an acceptable compromise?
> 
> Yeah that logic isn't very nice. I think it's a good idea to keep the
> tag depth at half the software depth, ie depth <= q->nr_requests. But
> I'll remove the auto-adjust.


Jens, are you saying that the host driver should enforce this limit?
Which auto-adjust are you removing?

thanks

jeremy