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Date:   Thu, 8 Sep 2022 11:12:20 +0200
From:   Jan Kara <jack@suse.cz>
To:     Ojaswin Mujoo <ojaswin@linux.ibm.com>
Cc:     Jan Kara <jack@suse.cz>, Ted Tso <tytso@mit.edu>,
        linux-ext4@vger.kernel.org,
        Thorsten Leemhuis <regressions@leemhuis.info>,
        Stefan Wahren <stefan.wahren@i2se.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>
Subject: Re: [PATCH 0/5 v2] ext4: Fix performance regression with mballoc
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On Thu 08-09-22 13:47:56, Ojaswin Mujoo wrote:
> On Tue, Sep 06, 2022 at 05:29:06PM +0200, Jan Kara wrote:
> > Hello,
> > 
> > Here is a second version of my mballoc improvements to avoid spreading
> > allocations with mb_optimize_scan=1. The patches fix the performance
> > regression I was able to reproduce with reaim on my test machine:
> > 
> >                      mb_optimize_scan=0     mb_optimize_scan=1     patched
> > Hmean     disk-1       2076.12 (   0.00%)     2099.37 (   1.12%)     2032.52 (  -2.10%)
> > Hmean     disk-41     92481.20 (   0.00%)    83787.47 *  -9.40%*    90308.37 (  -2.35%)
> > Hmean     disk-81    155073.39 (   0.00%)   135527.05 * -12.60%*   154285.71 (  -0.51%)
> > Hmean     disk-121   185109.64 (   0.00%)   166284.93 * -10.17%*   185298.62 (   0.10%)
> > Hmean     disk-161   229890.53 (   0.00%)   207563.39 *  -9.71%*   232883.32 *   1.30%*
> > Hmean     disk-201   223333.33 (   0.00%)   203235.59 *  -9.00%*   221446.93 (  -0.84%)
> > Hmean     disk-241   235735.25 (   0.00%)   217705.51 *  -7.65%*   239483.27 *   1.59%*
> > Hmean     disk-281   266772.15 (   0.00%)   241132.72 *  -9.61%*   263108.62 (  -1.37%)
> > Hmean     disk-321   265435.50 (   0.00%)   245412.84 *  -7.54%*   267277.27 (   0.69%)
> > 
> > The changes also significanly reduce spreading of allocations for small /
> > moderately sized files. I'm not able to measure a performance difference
> > resulting from this but on eMMC storage this seems to be the main culprit
> > of reduced performance. Untarring of raspberry-pi archive touches following
> > numbers of groups:
> > 
> > 	mb_optimize_scan=0	mb_optimize_scan=1	patched
> > groups	4			22			7
> > 
> > To achieve this I have added two more changes on top of v1 - patches 4 and 5.
> > Patch 4 makes sure we use locality group preallocation even for files that are
> > not likely to grow anymore (previously we have disabled all preallocations for
> > such files, however locality group preallocation still makes a lot of sense for
> > such files). This patch reduced spread of a small file allocations but larger
> > file allocations were still spread significantly because they avoid locality
> > group preallocation and as they are not power-of-two in size, they also
> > immediately start with cr=1 scan. To address that I've changed the data
> > structure for looking up the best block group to allocate from (see patch 5
> > for details).
> > 
> > Stefan, can you please test whether these patches fix the problem for you as
> > well? Comments & review welcome.
> > 
> > 								Honza
> > Previous versions:
> > Link: http://lore.kernel.org/r/20220823134508.27854-1-jack@suse.cz # v1
> 
> Hi Jan,
> 
> Thanks for the patch. I tested this series on my raspberry pi and I can
> confirm that the regression is no longer present with both
> mb_optimize_scan=0 and =1 taking similar amount of time to untar. The
> allocation spread I'm seeing is as follows:
> mb_optimize_scan=0: 10
> mb_optimize_scan=1: 17 (Check graphs for more details)
> 
> For mb_optimize_scan=1, I also compared the spread of locality group PA
> eligible files (<64KB) and inode pa files. The results can be found
> here:
> 
> mb_optimize_scan=0:
> https://github.com/OjaswinM/mbopt-bug/blob/master/grpahs/patchv2-mbopt0.png
> mb_optimize_scan=1:
> https://github.com/OjaswinM/mbopt-bug/blob/master/grpahs/patchv2.png
> mb_optimize_scan=1 (lg pa only):
> https://github.com/OjaswinM/mbopt-bug/blob/master/grpahs/patchv2-lgs.png
> mb_optimize_scan=1 (inode pa only):
> https://github.com/OjaswinM/mbopt-bug/blob/master/grpahs/patchv2-i.png
> 
> The smaller files are now closer together due to the changes to
> locality group pa logic. Most of the spread is now coming from mid to
> large files.
> 
> To test this further, I created a tar of 2000 100KB files to see if
> there is any performance drop due to higher spread of these files and
> notcied that although the spread is slightly higher(5BGs vs 9), we don't
> see a performance drop while untarring with mb_optimize_scan=1.
> 
> Although we still have some spread, I think we have brought it down to a
> much more manageable level and that combined with improvements to CR1
> allocation have given us a good performance improvement.
> 
> Feel free to add:
> Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>

Thanks a lot for the throughout testing!

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR