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

Andreas Dilger
2022-09-04T22:33:05Z

On Sep 4, 2022, at 00:01, Stefan Wahren <stefan.wahren@i2se.com> wrote:
>
>> Am 27.08.22 um 16:36 schrieb Ojaswin Mujoo:
>>> On Fri, Aug 26, 2022 at 12:15:22PM +0200, Jan Kara wrote:
>>> Hi Stefan,
>>>
>>> On Thu 25-08-22 18:57:08, Stefan Wahren wrote:
>>>>> 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
>>> Thanks! So now the iostat data indeed looks substantially different.
>>>
>>> nooptimize optimize
>>> Total written 183.6 MB 190.5 MB
>>> Time (recorded) 283 s 1040 s
>>> Avg write request size 79 KB 41 KB
>>>
>>> So indeed with mb_optimize_scan=1 we do submit substantially smaller
>>> requests on average. So far I'm not sure why that is. Since Ojaswin can
>>> reproduce as well, let's see what he can see from block location info.
>>> Thanks again for help with debugging this and enjoy your vacation!
>>>
>> Hi Jan and Stefan,
>>
>> Apologies for the delay, I was on leave yesterday and couldn't find time to get to this.
>>
>> So I was able to collect the block numbers using the method you suggested. I converted the
>> blocks numbers to BG numbers and plotted that data to visualze the allocation spread. You can
>> find them here:
>>
>> mb-opt=0, patched kernel: https://github.com/OjaswinM/mbopt-bug/blob/master/grpahs/mbopt-0-patched.png
>> mb-opt=1, patched kernel: https://github.com/OjaswinM/mbopt-bug/blob/master/grpahs/mbopt-1-patched.png
>> mb-opt=1, unpatched kernel: https://github.com/OjaswinM/mbopt-bug/blob/master/grpahs/mbopt-1-unpatched.png
>>
>> Observations:
>> * Before the patched mb_optimize_scan=1 allocations were way more spread out in
>> 40 different BGs.
>> * With the patch, we still allocate in 36 different BGs but majority happen in
>> just 1 or 2 BGs.
>> * With mb_optimize_scan=0, we only allocate in just 7 unique BGs, which could
>> explain why this is faster.
>
> thanks this is very helpful for me to understand. So it seems to me that with disabled mb_optimized_scan we have a more sequential write behavior and with enabled mb_optimized_scan a more random write behavior.
>
> From my understanding writing small blocks at random addresses of the sd card flash causes a lot of overhead, because the sd card controller need to erase huge blocks (up to 1 MB) before it's able to program the flash pages. This would explain why this series doesn't fix the performance issue, the total amount of BGs is still much higher.
>
> Is this new block allocation pattern a side effect of the optimization or desired?

The goal of the mb_optimized_scan is to avoid a large amount of linear scanning for
very large filesystems when there are many block groups that are full or fragmented.

It seems for empty filesystems the new list management is not very ideal. It probably
makes sense to have a hybrid, with some small amount of linear scanning (eg. a meta
block group worth), and then use the new list to find a new group if that doesn't find any
group with free space.

Cheers, Andreas