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

Stefan Wahren
2022-08-24T21:25:07Z

Hi Jan,

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

>
>> Unfortuntately i don't have much time this week and next week i'm in
>> holidays.
> No problem.
>
>> Just a question, my tests always had MBCACHE=y . Is it possible that the
>> mb_optimize_scan is counterproductive for MBCACHE in this case?
> MBCACHE (despite similar name) is actually related to extended attributes
> so it likely has no impact on your workload.
>
>> I'm asking because before the download the update script removes the files
>> from the previous update process which already cause a high load.
> Do you mean already the removal step is noticeably slower with
> mb_optimize_scan=1? The removal will be modifying directory blocks, inode
> table blocks, block & inode bitmaps, and group descriptors. So if block
> allocations are more spread (due to mb_optimize_scan=1 used during the
> untar), the removal may also take somewhat longer.
Not sure about this, maybe we should concentrate on download / untar phase.
> Honza