[PATCH 0/5 v3] ext4: Fix performance regression with mballoc

Jan Kara
2022-09-08T09:23:39Z

Hello,

Here is the third version of my mballoc improvements to avoid spreading
allocations with mb_optimize_scan=1. Since v2 there are only small changes and
fixes found during review and testing. Overall the series looks mostly ready to
go, I just didn't see any comments regarding patch 3 - a fix of metabg handling
in the Orlov allocator which is kind of independent, I've just found it when
reading the code. Also patch 5 needs final review after all the fixes.

Changes since v1:
- reworked data structure for CR 1 scan
- make small closed files use locality group preallocation
- fix metabg handling in the Orlov allocator

Changes since v2:
- whitespace fixes
- fix outdated comment
- fix handling of mb_structs_summary procfs file
- fix bad unlock on error recovery path

Original cover letter:

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).

Honza
Previous versions:
Link: http://lore.kernel.org/r/20220823134508.27854-1-jack@suse.cz # v1
Link: http://lore.kernel.org/r/20220906150803.375-1-jack@suse.cz # v2