[bug-report] 5-9% FIO randomwrite ext4 perf regression on 6.12.y kernel

[Jens Axboe]

On 11/20/24 5:00 PM, Chaitanya Kulkarni wrote:
> On 11/20/24 13:35, Saeed Mirzamohammadi wrote:
>> Hi,
>>
>> I?m reporting a performance regression of up to 9-10% with FIO randomwrite benchmark on ext4 comparing 6.12.0-rc2 kernel and v5.15.161. Also, standard deviation after this change grows up to 5-6%.
>>
>> Bisect root cause commit
>> ===================
>> - commit 63dfa1004322 ("nvme: move NVME_QUIRK_DEALLOCATE_ZEROES out of nvme_config_discard?)
>>
>>
>> Test details
>> =========
>> - readwrite=randwrite bs=4k size=1G ioengine=libaio iodepth=16 direct=1 time_based=1 ramp_time=180 runtime=1800 randrepeat=1 gtod_reduce=1
>> - Test is on ext4 filesystem
>> - System has 4 NVMe disks
>>
> 
> Thanks a lot for the report, to narrow down this problem can you
> please :-
> 
> 1. Run the same test on the raw nvme device /dev/nvme0n1 that you
>     have used for this benchmark ?
> 2. Run the same test on the  XFS formatted nvme device instead of ext4 ?
> 
> This way we will know if there is an issue only with the ext4 or
> with other file systems are suffering from this problem too or
> it is below the file system layer such as block layer and nvme pci driver ?
> 
> It will also help if you can repeat these numbers for io_uring fio io_engine
> to narrow down this problem to know if the issue is ioengine specific.
> 
> Looking at the commit [1], it only sets the max value to write zeroes 
> sectors
> if NVME_QUIRK_DEALLOCATE_ZEROES is set, else uses the controller max
> write zeroes value.

There's no way that commit is involved, the test as quoted doesn't even
touch write zeroes. Hence if there really is a regression here, then
it's either not easily bisectable, some error was injected while
bisecting, or the test itself is bimodal.

-- 
Jens Axboe