linux-ext4 - [PATCH 0/5 v2] ext4: Fix performance regression with mballoc

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Date:   Tue,  6 Sep 2022 17:29:06 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ted Tso <tytso@....edu>
Cc:     <linux-ext4@...r.kernel.org>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>
Subject: [PATCH 0/5 v2] ext4: Fix performance regression with mballoc

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