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

Jan Kara
2022-08-25T09:19:11Z

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@suse.com>
SUSE Labs, CR