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Date: Fri, 16 Oct 2009 17:45:05 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: john stultz <johnstul@...ibm.com> Cc: Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>, Nick Piggin <npiggin@...e.de>, Darren Hart <dvhltc@...ibm.com>, Clark Williams <williams@...hat.com>, Dinakar Guniguntala <dino@...ibm.com>, lkml <linux-kernel@...r.kernel.org> Subject: Re: -rt dbench scalabiltiy issue 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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