linux-ext4 - Re: [PATCH 0/2] ext4: Fix performance regression with mballoc

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Date:   Thu, 25 Aug 2022 11:18:42 +0200
From:   Jan Kara <jack@...e.cz>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>,
        linux-ext4@...r.kernel.org,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: Re: [PATCH 0/2] ext4: Fix performance regression with mballoc

Hi Stefan!

On Wed 24-08-22 23:24:43, Stefan Wahren wrote:
> Am 24.08.22 um 12:40 schrieb Jan Kara:
> > Hi Stefan!
> > 
> > On Wed 24-08-22 12:17:14, Stefan Wahren wrote:
> > > Am 23.08.22 um 22:15 schrieb Jan Kara:
> > > > Hello,
> > > > 
> > > > So I have implemented mballoc improvements to avoid spreading allocations
> > > > even with mb_optimize_scan=1. It fixes 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%)
> > > > 
> > > > Stefan, can you please test whether these patches fix the problem for you as
> > > > well? Comments & review welcome.
> > > i tested the whole series against 5.19 and 6.0.0-rc2. In both cases the
> > > update process succeed which is a improvement, but the download + unpack
> > > duration ( ~ 7 minutes ) is not as good as with mb_optimize_scan=0 ( ~ 1
> > > minute ).
> > OK, thanks for testing! I'll try to check specifically untar whether I can
> > still see some differences in the IO pattern on my test machine.
> 
> i made two iostat output logs during the complete download phase with 5.19
> and your series applied. iostat was running via ssh connection and
> rpi-update via serial console.
> 
> First with mb_optimize_scan=0
> 
> https://github.com/lategoodbye/mb_optimize_scan_regress/blob/main/5.19_SDCIT_patch_nooptimize_download_success.iostat.log
> 
> Second with mb_optimize_scan=1
> 
> https://github.com/lategoodbye/mb_optimize_scan_regress/blob/main/5.19_SDCIT_patch_optimize_download_success.iostat.log
> 
> Maybe this helps

Thanks for the data! So this is interesting. In both iostat logs, there is
initial phase where no IO happens. I guess that's expected. It is
significantly longer in the mb_optimize_scan=0 but I suppose that is just
caused by a difference in when iostat was actually started. Then in
mb_optimize_scan=0 there is 155 seconds where the eMMC card is 100%
utilized and then iostat ends. During this time ~63MB is written
altogether. Request sizes vary a lot, average is 60KB.

In mb_optimize_scan=1 case there is 715 seconds recorded where eMMC card is
100% utilized. During this time ~133MB is written, average request size is
40KB. If I look just at first 155 seconds of the trace (assuming iostat was
in both cases terminated before writing was fully done), we have written
~53MB and average request size is 56KB.

So with mb_optimize_scan=1 we are indeed still somewhat slower but based on
the trace it is not clear why the download+unpack should take 7 minutes
instead of 1 minute. There must be some other effect we are missing.

Perhaps if you just download the archive manually, call sync(1), and measure
how long it takes to (untar the archive + sync) in mb_optimize_scan=0/1 we
can see whether plain untar is indeed making the difference or there's
something else influencing the result as well (I have checked and
rpi-update does a lot of other deleting & copying as the part of the
update)? Thanks.

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