Re: io_request_lock patch?

[Jens Axboe <axboe@suse.de>]

On Wed, Jul 11 2001, Dipankar Sarma wrote:
> On Wed, Jul 11, 2001 at 04:01:48PM +0200, Jens Axboe wrote:
> > On Wed, Jul 11 2001, Dipankar Sarma wrote:
> > > On Wed, Jul 11, 2001 at 10:53:39AM +0200, Jens Axboe wrote:
> > > > The queue lengths should always be long enough to keep the hw busy of
> > > > course. And in addition, the bigger the queues the bigger the chance of
> > > > skipping seeks due to reordering. But don't worry, I've scaled the queue
> > > > lengths so I'm pretty sure that they are always on the safe side in
> > > > size.
> > > > 
> > > > It's pretty easy to test for yourself if you want, just change
> > > > QUEUE_NR_REQUESTS in blkdev.h. It's currently 8192, the request slots
> > > > are scaled down from this value. 8k will give you twice the amount of
> > > > slots that you have RAM in mb, ie 2048 on a 1gig machine.
> > > > 
> > > > block: queued sectors max/low 683554kB/552482kB, 2048 slots per queue
> > > 
> > > Hmm.. The tiobench run was done on a 1GB machine and we still ran
> > > out of request slots. Will investigate.
> > 
> > Sure, that's to be expected. If we never ran out we would be wasting
> > memory. My point is that you should rerun the same test with more
> > request slots -- and I'd be surprised if you gained any significant
> > performance on that account. I never said that you'd never run out,
> > that's of course not true. In fact, running out is what starts the I/O
> > typically on a 1GB machine and bigger.
> 
> I agree that running out of slots don't necessarily mean anything. I
> was just wondering about the queue lengths. On the face of it, 2048 
> requests pending for a single disk seems like a fairly large queue length.
> Is this all related to plugging of queues for disk block optimization ?

Plugging to a degree, although when you get queues this long you can
get nice merging even though the queue will probably never get replugged
(because it never empties). The merging is the most important step, and
also the insertion sort of course.

-- 
Jens Axboe