Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754227Ab0G0NSb (ORCPT ); Tue, 27 Jul 2010 09:18:31 -0400 Received: from bld-mail19.adl2.internode.on.net ([150.101.137.104]:59559 "EHLO mail.internode.on.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753181Ab0G0NS3 (ORCPT ); Tue, 27 Jul 2010 09:18:29 -0400 Date: Tue, 27 Jul 2010 23:18:10 +1000 From: Dave Chinner To: Nick Piggin Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Frank Mayhar , John Stultz Subject: Re: VFS scalability git tree Message-ID: <20100727131810.GO7362@dastard> References: <20100722190100.GA22269@amd> <20100723135514.GJ32635@dastard> <20100727070538.GA2893@amd> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100727070538.GA2893@amd> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7025 Lines: 160 On Tue, Jul 27, 2010 at 05:05:39PM +1000, Nick Piggin wrote: > On Fri, Jul 23, 2010 at 11:55:14PM +1000, Dave Chinner wrote: > > On Fri, Jul 23, 2010 at 05:01:00AM +1000, Nick Piggin wrote: > > > I'm pleased to announce I have a git tree up of my vfs scalability work. > > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/npiggin/linux-npiggin.git > > > http://git.kernel.org/?p=linux/kernel/git/npiggin/linux-npiggin.git > > > > > > Branch vfs-scale-working > > > > With a production build (i.e. no lockdep, no xfs debug), I'll > > run the same fs_mark parallel create/unlink workload to show > > scalability as I ran here: > > > > http://oss.sgi.com/archives/xfs/2010-05/msg00329.html > > I've made a similar setup, 2s8c machine, but using 2GB ramdisk instead > of a real disk (I don't have easy access to a good disk setup ATM, but > I guess we're more interested in code above the block layer anyway). > > Made an XFS on /dev/ram0 with 16 ags, 64MB log, otherwise same config as > yours. A s a personal prefernce, I don't like testing filesystem performance on ramdisks because it hides problems caused by changes in IO latency. I'll come back to this later. > I found that performance is a little unstable, so I sync and echo 3 > > drop_caches between each run. Quite possibly because of the smaller log - that will cause more frequent pushing on the log tail and hence I/O patterns will vary a bit... Also, keep in mind that delayed logging is shiny and new - it has increased XFS metadata performance and parallelism by an order of magnitude and so we're really seeing new a bunch of brand new issues that have never been seen before with this functionality. As such, there's still some interactions I haven't got to the bottom of with delayed logging - it's stable enough to use and benchmark and won't corrupt anything but there are still has some warts we need to solve. The difficulty (as always) is in reliably reproducing the bad behaviour. > When it starts reclaiming memory, things > get a bit more erratic (and XFS seemed to be almost livelocking for tens > of seconds in inode reclaim). I can't say that I've seen this - even when testing up to 10m inodes. Yes, kswapd is almost permanently active on these runs, but when creating 100,000 inodes/s we also need to be reclaiming 100,000 inodes/s so it's not surprising that when 7 CPUs are doing allocation we need at least one CPU to run reclaim.... > So I started with 50 runs of fs_mark > -n 20000 (which did not cause reclaim), rebuilding a new filesystem > between every run. > > That gave the following files/sec numbers: > N Min Max Median Avg Stddev > x 50 100986.4 127622 125013.4 123248.82 5244.1988 > + 50 100967.6 135918.6 130214.9 127926.94 6374.6975 > Difference at 95.0% confidence > 4678.12 +/- 2316.07 > 3.79567% +/- 1.87919% > (Student's t, pooled s = 5836.88) > > This is 3.8% in favour of vfs-scale-working. > > I then did 10 runs of -n 20000 but with -L 4 (4 iterations) which did > start to fill up memory and cause reclaim during the 2nd and subsequent > iterations. I haven't used this mode, so I can't really comment on the results you are seeing. > > enabled. ext4 is using default mkfs and mount parameters except for > > barrier=0. All numbers are averages of three runs. > > > > fs_mark rate (thousands of files/second) > > 2.6.35-rc5 2.6.35-rc5-scale > > threads xfs ext4 xfs ext4 > > 1 20 39 20 39 > > 2 35 55 35 57 > > 4 60 41 57 42 > > 8 79 9 75 9 > > > > ext4 is getting IO bound at more than 2 threads, so apart from > > pointing out that XFS is 8-9x faster than ext4 at 8 thread, I'm > > going to ignore ext4 for the purposes of testing scalability here. > > > > For XFS w/ delayed logging, 2.6.35-rc5 is only getting to about 600% > > CPU and with Nick's patches it's about 650% (10% higher) for > > slightly lower throughput. So at this class of machine for this > > workload, the changes result in a slight reduction in scalability. > > I wonder if these results are stable. It's possible that changes in > reclaim behaviour are causing my patches to require more IO for a > given unit of work? More likely that's the result of using a smaller log size because it will require more frequent metadata pushes to make space for new transactions. > I was seeing XFS 'livelock' in reclaim more with my patches, it > could be due to more parallelism now being allowed from the vfs and > reclaim. > > Based on my above numbers, I don't see that rcu-inodes is causing a > problem, and in terms of SMP scalability, there is really no way that > vanilla is more scalable, so I'm interested to see where this slowdown > is coming from. As I said initially, ram disks hide IO latency changes resulting from increased numbers of IO or increases in seek distances. My initial guess is the change in inode reclaim behaviour causing different IO patterns and more seeks under reclaim because the zone based reclaim is no longer reclaiming inodes in the order they are created (i.e. we are not doing sequential inode reclaim any more. FWIW, I use PCP monitoring graphs to correlate behavioural changes across different subsystems because it is far easier to relate information visually than it is by looking at raw numbers or traces. I think this graph shows the effect of relcaim on performance most clearly: http://userweb.kernel.org/~dgc/shrinker-2.6.36/fs_mark-2.6.35-rc3-context-only-per-xfs-batch6-16x500-xfs.png It's pretty clear that when the inode/dentry cache shrinkers are running, sustained create/unlink performance goes right down. From a different tab not in the screen shot (the other "test-4" tab), I could see CPU usage also goes down and the disk iops go way up whenever the create/unlink performance dropped. This same behaviour happens with the vfs-scale patchset, so it's not related to lock contention - just aggressive reclaim of still-dirty inodes. FYI, The patch under test there was the XFS shrinker ignoring 7 out of 8 shrinker calls and then on the 8th call doing the work of all previous calls. i.e emulating SHRINK_BATCH = 1024. Interestingly enough, that one change reduced the runtime of the 8m inode create/unlink load by ~25% (from ~24min to ~18min). That is by far the largest improvement I've been able to obtain from modifying the shrinker code, and it is from those sorts of observations that I think that IO being issued from reclaim is currently the most significant performance limiting factor for XFS in this sort of workload.... Cheers, Dave. -- Dave Chinner david@fromorbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/