Re: Performance regressions in 2.6.30-rc7?

[Jeff Moyer]

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.

Thanks for looking into this!

-Jeff
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