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

Stefan Wahren
2022-08-25T16:57:24Z

Hi Jan,

Am 25.08.22 um 11:18 schrieb Jan Kara:
> 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.

mb_optimize_scan=0 -> almost 5 minutes

mb_optimize_scan=1 -> almost 18 minutes

https://github.com/lategoodbye/mb_optimize_scan_regress/commit/3f3fe8f87881687bb654051942923a6b78f16dec

>
> Honza