Re: IO scheduler, queue depth, nr_requests

[Jens Axboe <axboe@suse.de>]

On Thu, Feb 19 2004, Nick Piggin wrote:
> 
> 
> Miquel van Smoorenburg wrote:
> 
> >
> >No, I'm actually referring to a struct request. I'm logging this in the
> >SCSI layer, in scsi_request_fn(), just after elv_next_request(). I have
> >in fact logged all the bio's submitted to __make_request, and the output
> >of the elevator from elv_next_request(). The bio's are submitted 
> >sequentially,
> >the resulting requests aren't. But this is because nr_requests is 128, 
> >while
> >the 3ware device has a queue of 254 entries (no tagging though). Upping
> >nr_requests to 512 makes this go away ..
> >
> >That shouldn't be necessary though. I only see this with LVM over 
> >3ware-raid5,
> >not on the 3ware-raid5 array directly (/dev/sda1). And it gets less 
> >troublesome
> >with a lot of debugging (unless I set nr_requests lower again), which 
> >points
> >to a timing issue.
> >
> >
> 
> So the problem you are seeing is due to "unlucky" timing between
> two processes submitting IO. And the very efficient mechanisms
> (merging, sorting) we have to improve situations exactly like this
> is effectively disabled. And to make it worse, it appears that your
> controller shits itself on this trivially simple pattern.
> 
> Your hack makes a baby step in the direction of per *process*
> request limits, which I happen to be an advocate of. As it stands
> though, I don't like it.

I'm very much an advocate for per process request limits as well.  Would
be trivial to add... Miquels patch is horrible, I appreciate it being
posted as a cry for help.

-- 
Jens Axboe