Re: -rt dbench scalabiltiy issue

[john stultz <johnstul@us.ibm.com>]

On Fri, 2009-10-16 at 17:45 -0700, Paul E. McKenney wrote:
> On Fri, Oct 16, 2009 at 01:05:19PM -0700, john stultz wrote:
> > See http://lwn.net/Articles/354690/ for a bit of background here.
> > 
> > I've been looking at scalability regressions in the -rt kernel. One easy
> > place to see regressions is with the dbench benchmark. While dbench can
> > be painfully noisy from run to run, it does clearly show some severe
> > regressions with -rt. 
> > 
> > There's a chart in the article above that illustrates this, but here's
> > some specific numbers on an 8-way box running dbench-3.04 as follows: 
> > 
> > ./dbench 8 -t 10 -D . -c client.txt 2>&1 
> > 
> > I ran both on an ext3 disk and a ramfs mounted directory.
> > 
> > (Again, the numbers are VERY rough due to the run-to-run variance seen)
> > 
> > 			ext3		ramfs 
> > 2.6.32-rc3:		~1800 MB/sec	~1600 MB/sec
> > 2.6.31.2-rt13: 		~300 MB/sec	~66 MB/sec
> > 
> > Ouch. Similar to the charts in the LWN article.
> > 
> > Dino pointed out that using lockstat with -rt, we can see the
> > dcache_lock is fairly hot with the -rt kernel. One of the issues with
> > the -rt tree is that the change from spinlocks to sleeping-spinlocks
> > doesn't effect the un-contended case very much, but when there is
> > contention on the lock, the overhead is much worse then with vanilla.
> > 
> > And as noted at the realtime mini-conf, Ingo saw this dcache_lock
> > bottleneck as well and suggested trying Nick Piggin's dcache_lock
> > removal patches.
> > 
> > So over the last week, I've ported Nick's fs-scale patches to -rt. 
> > 
> > Specifically the tarball found here:
> > ftp://ftp.kernel.org/pub/linux/kernel/people/npiggin/patches/fs-scale/06102009.tar.gz
> > 
> > 
> > Due to the 2.6.32 2.6.31-rt split, the port wasn't exactly straight
> > forward, but I believe I managed to do a decent job. Once I had the
> > patchset applied, building and booted, I eagerly ran dbench to see the
> > new results, aaaaaand.....
> > 
> > 			ext3		ramfs 
> > 2.6.31.2-rt13-nick:	~80 MB/sec	~126 MB/sec
> > 
> > 
> > So yea, mixed bag there. The ramfs got a little bit better but not that
> > much, and the ext3 numbers regressed further.
> 
> OK, I will ask the stupid question...  What happens if you run on ext2?

Yep. That was next on my list. Basically its faster, but the regressions
are similar % wise with each patchset.

			ext3		ext2
2.6.32-rc3:		~1800 MB/sec	~2900 MB/sec
2.6.31.2-rt13:		~300 MB/sec	~600 MB/sec
2.6.31.2-rt13-nick:	~80 MB/sec	~130 MB/sec

thanks
-john