Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

[Paul Clements]

Jens Axboe wrote:
> On Fri, Feb 08 2008, Jens Axboe wrote:
> > On Fri, Feb 08 2008, Paul Clements wrote:
> > > Andrew Morton wrote:
> > > > On Fri, 8 Feb 2008 09:33:41 -0800 Randy Dunlap <randy.dunlap@xxxxxxxxxx> 
> > > > wrote:
> > > >
> > > > > On Fri, 08 Feb 2008 11:47:42 -0500 Paul Clements wrote:
> > > > >
> > > > > > There have been numerous reports of problems with nbd and cfq. Deadline 
> > > > > > gives better performance for nbd, anyway, so let's use it by default.
> > > > Please define "problems".  If it's just "slowness" then we can live with
> > > > that, but I'd hope that Jens is aware and that it's understood.
> > > >
> > > > It it's "hangs" or "oopses" then we panic.
> > > The two problems I have experienced (which may already be fixed):
> > >
> > > 1) nbd hangs with cfq on RHEL 5 (2.6.18) -- this may well have been fixed
> > >
> > > There's a similar debian bug that has been filed as well:
> > >
> > > http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=447638
> > >
> > > 2) nbd performs about 10% better (the last time I tested) with deadline 
> > > vs. cfq (the overhead of cfq doesn't provide much advantage to nbd [not 
> > > being a real disk], and you end up going through the I/O scheduler on 
> > > the nbd server anyway, so it makes sense that deadline is better with nbd)
> > >
> > > There have been posts to nbd-general mailing list about problems with 
> > > cfq and nbd also.
> > I'm fine with that, it's one of those things we'll do automatically when
> > we have some sort of disk profile system setup. Devices without seek
> > penalties should not use AS or CFQ.
> >
> > Asking for a non-existing elevator is not an issue, but it may trigger
> > both printks and a switch to another elevator. So if you ask for
> > "deadline" and it's modular, you'll get cfq again if it's the default.
> >
> > Your patch looks bad though, you forget to exit the old elevator. And
> > you don't check the return value of elevator_init().
> >
> > All in all, your patch definitely needs more work before it can be
> > included.

Thanks Jens. This one should be better.

--
Paul
NBD doesn't work well with CFQ (or AS) schedulers, so let's default to something
else.

The two problems I have experienced with nbd and cfq are:
 
1) nbd hangs with cfq on RHEL 5 (2.6.18) -- this may well have been fixed
 
There's a similar debian bug that has been filed as well:
 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=447638
 
2) nbd performs about 10% better (the last time I tested) with deadline 
vs. cfq (the overhead of cfq doesn't provide much advantage to nbd [not 
being a real disk], and you end up going through the I/O scheduler on 
the nbd server anyway, so it makes sense that deadline is better with nbd)
 
There have been posts to nbd-general mailing list about problems with 
cfq and nbd also.

Signed-Off-By: Paul Clements <paul.clements@xxxxxxxxxxxx>

--- ./drivers/block/nbd.c.max_nbd_killed	2008-02-07 16:46:24.000000000 -0500
+++ ./drivers/block/nbd.c	2008-02-08 16:13:01.000000000 -0500
@@ -667,6 +667,12 @@ static int __init nbd_init(void)
 			put_disk(disk);
 			goto out;
 		}
+		if (elevator_init(disk->queue, "deadline") != 0) {
+			if (elevator_init(disk->queue, "noop") != 0) {
+				put_disk(disk);
+				goto out;
+			}
+		}
 	}
 
 	if (register_blkdev(NBD_MAJOR, "nbd")) {