Re: -rt dbench scalabiltiy issue

[Paul E. McKenney]

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?

							Thanx, Paul

> I then looked into the perf tool, to see if it would shed some light on
> whats going on (snipped results below).
> 
> 2.6.31.2-rt13 on ext3: 
> 42.45%   dbench  [kernel]                    [k] _atomic_spin_lock_irqsave
>                 |          
>                 |--85.61%-- rt_spin_lock_slowlock
>                 |          rt_spin_lock
>                 |          |          
>                 |          |--23.91%-- start_this_handle
>                 |          |          journal_start
>                 |          |          ext3_journal_start_sb
>                 |          |--21.29%-- journal_stop
>                 |          |
>                 |          |--13.80%-- ext3_test_allocatable
>                 |          |          
>                 |          |--12.15%-- bitmap_search_next_usable_block
>                 |          |          
>                 |          |--9.79%-- journal_put_journal_head
>                 |          |          
>                 |          |--5.93%-- journal_add_journal_head
>                 |          |          
>                 |          |--2.59%-- atomic_dec_and_spin_lock
>                 |          |          dput
>                 |          |          |          
>                 |          |          |--65.31%-- path_put
>                 |          |          |          |          
>                 |          |          |          |--53.37%-- __link_path_walk
> ...
> 
> So this is initially interesting, as it seems on ext3 it seems the
> journal locking is really whats catching us more then the dcache_lock.
> Am I reading this right? 
> 
> 
> 2.6.31.2-rt13 on ramfs: 
> 45.98%         dbench  [kernel]                    [k] _atomic_spin_lock_irqsave
>                 |          
>                 |--82.94%-- rt_spin_lock_slowlock
>                 |          rt_spin_lock
>                 |          |          
>                 |          |--61.18%-- dcache_readdir
>                 |          |          vfs_readdir
>                 |          |          sys_getdents
>                 |          |          system_call_fastpath
>                 |          |          __getdents64
>                 |          |          
>                 |          |--11.26%-- atomic_dec_and_spin_lock
>                 |          |          dput
>                 |          |          
>                 |          |--7.93%-- d_path
>                 |          |          seq_path
>                 |          |          show_vfsmnt
>                 |          |          seq_read
>                 |          |          vfs_read
>                 |          |          sys_read
>                 |          |          system_call_fastpath
>                 |          |          __GI___libc_read
>                 |          |          
> 
> 
> So here we do see the dcache_readdir's use of the dcache lock pop up to
> the top. And with ramfs we don't see any of the ext3 journal code.
> 
> Next up is with Nick's patchset:
> 
> 2.6.31.2-rt13-nick on ext3:
>     45.48%         dbench  [kernel]                    [k] _atomic_spin_lock_irqsave
>                 |          
>                 |--83.40%-- rt_spin_lock_slowlock
>                 |          |          
>                 |          |--100.00%-- rt_spin_lock
>                 |          |          |          
>                 |          |          |--43.35%-- dput
>                 |          |          |          |          
>                 |          |          |          |--50.29%-- __link_path_walk
>                 |          |          |           --49.71%-- path_put
>                 |          |          |--39.07%-- path_get
>                 |          |          |          |          
>                 |          |          |          |--61.98%-- path_walk
>                 |          |          |          |--38.01%-- path_init
>                 |          |          |          
>                 |          |          |--7.33%-- journal_put_journal_head
>                 |          |          |          
>                 |          |          |--4.32%-- journal_add_journal_head
>                 |          |          |          
>                 |          |          |--2.83%-- start_this_handle
>                 |          |          |          journal_start
>                 |          |          |          ext3_journal_start_sb
>                 |          |          |          
>                 |          |          |--2.52%-- journal_stop
>                 |          
>                 |--15.87%-- rt_spin_lock_slowunlock
>                 |          rt_spin_unlock
>                 |          |          
>                 |          |--43.48%-- path_get
>                 |          |          
>                 |          |--41.80%-- dput
>                 |          |          
>                 |          |--5.34%-- journal_add_journal_head
> 		...
> 
> With Nick's patches on ext3, it seems dput()'s locking is the bottleneck
> more then the journal code (maybe due to the multiple spinning nested
> trylocks?). 
> 
> With the ramfs, it looks mostly the same, but without the journal calls:
> 
> 2.6.31.2-rt13-nick on ramfs:
>     46.51%         dbench  [kernel]                  [k] _atomic_spin_lock_irqsave
>                 |          
>                 |--86.95%-- rt_spin_lock_slowlock
>                 |          rt_spin_lock
>                 |          |          
>                 |          |--50.08%-- dput
>                 |          |          |          
>                 |          |          |--56.92%-- __link_path_walk
>                 |          |          |          
>                 |          |           --43.08%-- path_put
>                 |          |          
>                 |          |--49.12%-- path_get
>                 |          |          |          
>                 |          |          |--63.22%-- path_walk
>                 |          |          |          
>                 |          |          |--36.73%-- path_init
>                 |          
>                 |--12.59%-- rt_spin_lock_slowunlock
>                 |          rt_spin_unlock
>                 |          |          
>                 |          |--49.86%-- path_get
>                 |          |          |          
>                 |          |          |--58.15%-- path_init
>                 |          |          |          |          
> ...
> 
> 
> So the net of this is: Nick's patches helped some but not that much in
> ramfs filesystems, and hurt ext3 performance w/ -rt.
> 
> Maybe I just mis-applied the patches? I'll admit I'm unfamiliar with the
> dcache code, and converting the patches to the -rt tree was not always
> straight forward.
> 
> Or maybe these results are expected? With Nick's patch against
> 2.6.32-rc3 I got:
> 
> 			ext3		ramfs 
> 2.6.32-rc3-nick		~1800 MB/sec	~2200 MB/sec
> 
> So ext3 performance didn't change, but ramfs did see a nice bump. Maybe
> Nick's patches helped where they could, but we still have other
> contention points that are problematic with -rt's lock slowpath
> overhead?
> 
> 
> Ingo, Nick, Thomas: Any thoughts or comments here? Am I reading perf's
> results incorrectly? Any idea why with Nick's patch the contention in
> dput() hurts ext3 so much worse then in the ramfs case?
> 
> 
> I'll be doing some further tests today w/ ext2 to see if getting the
> journal code out of the way shows any benefit. But if folks have any
> insight or suggestions for other ideas to look at please let me know.
> 
> thanks
> -john
> 
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