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Message-ID: <20090715175028.GE22826@atrey.karlin.mff.cuni.cz> Date: Wed, 15 Jul 2009 19:50:28 +0200 From: Jan Kara <jack@...e.cz> To: Jeff Moyer <jmoyer@...hat.com> Cc: Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>, Mike Galbraith <efault@....de>, Diego Calleja <diegocg@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, LKML <linux-kernel@...r.kernel.org>, jens.axboe@...cle.com, linux-ext4@...r.kernel.org Subject: Re: Performance regressions in 2.6.30-rc7? > On Wed 15-07-09 09:41:02, Jeff Moyer wrote: > > Jan Kara <jack@...e.cz> writes: > > > > > On Wed 10-06-09 18:12:50, Jeff Moyer wrote: > > >> Jan Kara <jack@...e.cz> writes: > > >> > > >> > On Tue 09-06-09 14:48:18, Chris Mason wrote: > > >> >> On Tue, Jun 09, 2009 at 12:32:08PM +0200, Jan Kara wrote: > > >> >> > On Thu 04-06-09 21:13:15, Mike Galbraith wrote: > > >> >> > > On Thu, 2009-06-04 at 13:21 +0200, Jan Kara wrote: > > >> >> > > > > >> >> > > > > Sequential Writes > > >> >> > > > > 2.6.30-smp-ordered 6000 65536 32 50.16 508.9% 31.996 45595.78 0.64965 0.02402 10 > > >> >> > > > > 2.6.29.4-smp-ordered 6000 65536 32 52.70 543.2% 33.658 23794.92 0.71754 0.00836 10 > > >> >> > > > > > > >> >> > > > > 2.6.30-smp-writeback 6000 65536 32 47.82 525.4% 35.003 32588.84 0.56192 0.02298 9 > > >> >> > > > > 2.6.29.4-smp-writeback 6000 65536 32 52.52 467.6% 32.397 12972.78 0.53580 0.00522 11 > > >> >> > > > > > > >> >> > > > > 2.6.30-smp-ordered 6000 65536 16 56.08 254.9% 15.463 33000.68 0.39687 0.00521 22 > > >> >> > > > > 2.6.29.4-smp-ordered 6000 65536 16 62.40 308.4% 14.701 13455.02 0.13125 0.00208 20 > > >> >> > > > > > > >> >> > > > > 2.6.30-smp-writeback 6000 65536 16 51.90 281.4% 17.098 12869.85 0.36771 0.00104 18 > > >> >> > > > > 2.6.29.4-smp-writeback 6000 65536 16 60.53 272.6% 14.977 8637.08 0.21146 0.00000 22 > > >> >> > > > > > > >> >> > > > > 2.6.30-smp-ordered 6000 65536 8 51.09 113.4% 8.700 14856.55 0.06771 0.00417 45 > > >> >> > > > > 2.6.29.4-smp-ordered 6000 65536 8 56.13 130.6% 8.098 8400.45 0.03958 0.00000 43 > > >> >> > > > > > > >> >> > > > > 2.6.30-smp-writeback 6000 65536 8 50.19 131.7% 8.680 16821.04 0.11979 0.00208 38 > > >> >> > > > > 2.6.29.4-smp-writeback 6000 65536 8 54.90 130.7% 8.244 4925.48 0.10000 0.00000 42 > > >> >> > > > It really seems write has some problems... There's consistently lower > > >> >> > > > throughput and it also seems some writes take really long. I'll try to > > >> >> > > > reproduce it here. > > >> >> > > > > >> >> > > Looked "pretty solid" to me. I haven't observed enough to ~trust. > > >> >> > OK, I did a few runs of tiobench here and I can confirm that I see about > > >> >> > 6% performance regression in Sequential Write throughput between 2.6.29 > > >> >> > and 2.6.30-rc8. I'll try to find what's causing it. > > >> >> > > >> >> My first guess would be the WRITE_SYNC style changes. Is the regression > > >> >> still there with noop? > > >> > Thanks for the hint. I was guessing that as well. And experiments show > > >> > it's definitely connected. To be more precise with the data: > > >> > The test machine is 2 CPU, 2 GB ram, simple lowend SATA disk. Tiobench run > > >> > with: > > >> > tiobench/tiobench.pl -b 65536 -t 16 -t 8 -d /local/scratch -s 4096 > > >> > which means 4GB testfile, writes happen in 64k chunks, test done with 16 > > >> > and 8 threads. /local/scratch is a separate partition always cleaned and > > >> > umounted + mounted before each test. The results are (always 3 runs): > > >> > 2.6.29+CFQ: Avg StdDev > > >> > 8 38.01 40.26 39.69 -> 39.32 0.955092 > > >> > 16 40.09 38.18 40.05 -> 39.44 0.891104 > > >> > > > >> > 2.6.30-rc8+CFQ: > > >> > 8 36.67 36.81 38.20 -> 37.23 0.69062 > > >> > 16 37.45 36.47 37.46 -> 37.13 0.464351 > > >> > > > >> > 2.6.29+NOOP: > > >> > 8 38.67 38.66 37.55 -> 38.29 0.525632 > > >> > 16 39.59 39.15 39.19 -> 39.31 0.198662 > > >> > > > >> > 2.6.30-rc8+NOOP: > > >> > 8 38.31 38.47 38.16 -> 38.31 0.126579 > > >> > 16 39.08 39.25 39.13 -> 39.15 0.0713364 > > >> > > >> I ran the same test on a bigger system: 8GB ram (so I used a 16GB size > > >> for the test) and a 4 disk stripe hanging off of a CCISS controller. > > >> All the runs used ext3 in data=ordered mode and CFQ as the I/O scheduler. > > >> > > >> 2.6.29.3-140.fc11 Avg StdDev > > >> 8 158.72 152.72 148.24 153.227 5.25834 > > >> 16 176.06 174.91 176.27 175.747 0.73214 > > >> > > >> 2.6.30-rc7 > > >> 8 147.89 144.57 144.99 145.817 1.8078 > > >> 16 121.37 119.56 111.85 117.593 5.05553 > > >> > > >> Jan, let me know if you want any help tracking this down. > > > OK, so I've found time to follow-up on this. I've checked that > > > congestion_wait fixes Jens sent recently didn't change anything. Also I've > > > verified that backing out WRITE_SYNC related changes didn't help. Finally, > > > I've verified that when I back out all the changes that went to CFQ between > > > 2.6.29 and 2.6.30 and the WRITE_SYNC changes, then the performance is back > > > to original values. > > > Jens / Jeff, what to do next? I can try to bisect through CFQ changes but > > > that's going to be rather tedious and the result is uncertain since I > > > expect performance to jump up and down as various changes took place. So > > > I'd rather spend my time with something that has a higher chance to > > > succeed... > > > > > > > Looking through the changelogs, I most suspect this: > > > > commit 2f5cb7381b737e24c8046fd4aeab571fb71315f5 > > Author: Jens Axboe <jens.axboe@...cle.com> > > Date: Tue Apr 7 08:51:19 2009 +0200 > > > > cfq-iosched: change dispatch logic to deal with single requests at > > the time > > > > We had one other regression that bisected to this change, though I don't > > claim to fully understand why just yet. Take a look at this bug: > > http://bugzilla.kernel.org/show_bug.cgi?id=13401 > > > > Try Jens' test patch posted there: > > http://bugzilla.kernel.org/attachment.cgi?id=21650 > > > > and let us know how that fares. > It seems that with this test patch, the throughput is somewhere between > 2.6.29 and 2.6.30. I'm now repeating runs more times to get more > statistical reliability because with 3 runs I did so far it's somewhere on > the boundary of statistical meaningfulness... OK, I did 7 runs from each test with 8 tiobench threads only. The results are: kernel avg 99%-reliability-interval 2.6.29 39.797143 0.860581 2.6.30-rc8 37.441429 0.632984 2.6.30-rc8+patch 37.538571 0.872624 Where the 99%-reliability-interval is the interval in which "real throughput" lies with 99% reliability (I did some studying of t-tests on Wikipedia ;). So a conclusion is that Jens's test patch didn't change anything. I guess I'll now try your patch from the referenced bug. Honza -- Jan Kara <jack@...e.cz> SuSE CR Labs -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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