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

[Jens Axboe <axboe@suse.de>]

On Tue, Sep 28 2004, Andrew Vasquez wrote:
> On Tue, 28 Sep 2004, Jens Axboe wrote:
> > On Tue, Sep 28 2004, James Bottomley wrote:
> > > On Tue, 2004-09-28 at 15:36, Jeremy Higdon wrote:
> > > > I was trying to be conservative -- I didn't know the reason for
> > > > that limit.  By all means, let's remove it then.
> > > > 
> > > > Would you like me to regenerate the patch, or do you just want
> > > > to apply it to your internal source that will eventually be
> > > > fed back to us?
> > > 
> > > 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 Axboe