Re: io_request_lock patch?

Dipankar Sarma
2001-07-11T14:51:20Z

Hi Jens,

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 ?

Thanks
Dipankar
--
Dipankar Sarma <dipankar@sequent.com> Project: http://lse.sourceforge.net
Linux Technology Center, IBM Software Lab, Bangalore, India.