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

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Message-ID: <20220907104947.fwbmewmgbnkug6dl@quack3>
Date:   Wed, 7 Sep 2022 12:49:47 +0200
From:   Jan Kara <jack@...e.cz>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Ted Tso <tytso@....edu>, Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: [PATCH 0/5 v2] ext4: Fix performance regression with mballoc

Hi Stefan!

On Tue 06-09-22 22:38:10, Stefan Wahren wrote:
> Am 06.09.22 um 17:29 schrieb Jan Kara:
> > 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.
> 
> this looks amazing \o/
> 
> With this patch v2 applied the untar with mb_optimize_scan=1 is now faster
> than mb_optimize_scan=0.
> 
> mb_optimize_scan=0 -> almost 5 minutes
> 
> mb_optimize_scan=1 -> almost 1 minute
> 
> The original scenario (firmware download) with mb_optimize_scan=1 is now
> fast as mb_optimize_scan=0.

Glad to hear that!

> Here the iostat as usual:
> 
> https://github.com/lategoodbye/mb_optimize_scan_regress/commit/f4ad188e0feee60bffa23a8e1ad254544768c3bd
> 
> There is just one thing, but not sure this if this comes from these patches.
> If i call
> 
> cat /proc/fs/ext4/mmcblk1p2/mb_structs_summary
> 
> The kernel throw a NULL pointer derefence in
> ext4_mb_seq_structs_summary_show

Yeah, likely a bug in my last patch. I didn't test my changes to the sysfs
interface. Thanks for testing this, I'll have a look.

								Honza

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