Re: Performance regressions in 2.6.30-rc7?

From: Jan Kara
Date: Wed Jul 15 2009 - 13:52:39 EST

> On Wed 15-07-09 09:41:02, Jeff Moyer wrote:
> > Jan Kara <jack@xxxxxxx> writes:
> >
> > > On Wed 10-06-09 18:12:50, Jeff Moyer wrote:
> > >> Jan Kara <jack@xxxxxxx> 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@xxxxxxxxxx>
> > 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@xxxxxxx>
SuSE CR Labs
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