Re: [BUG] 500-2000% performance regression w/ 5.10

[Nikolay Borisov <nborisov@suse.com>]

On 24.12.20 г. 23:11 ч., René Rebe wrote:
> Hi Josef,
> 
>> On 24. Dec 2020, at 19:09, Josef Bacik <josef@toxicpanda.com> wrote:
>>
>> On 12/21/20 2:45 PM, René Rebe wrote:
>>> Hey there,
>>> as a long time btrfs user I noticed some some things became very slow
>>> w/ Linux kernel 5.10. I found a very simple test case, namely extracting
>>> a huge tarball like:
>>>   tar xf /usr/src/t2-clean/download/mirror/f/firefox-84.0.source.tar.zst
>>> Why my external, USB3 road-warrior SSD on a Ryzen 5950x this
>>> went from ~15 seconds w/ 5.9 to nearly 5 minutes in 5.10, or 2000%
>>> To rule out USB, I also tested a brand new PCIe 4.0 SSD, with
>>> a similar, albeit not as shocking regression from 5.2 seconds
>>> to ~34 seconds or∫~650%.
>>> Somehow testing that in a VM did over virtio did not produce
>>> as different results, although it was already 35 seconds slow
>>> with 5.9.
>>> # first bad commit: [38d715f494f2f1dddbf3d0c6e50aefff49519232]
>>>   btrfs: use btrfs_start_delalloc_roots in shrink_delalloc
>>> Now just this single commit does obviously not revert cleanly,
>>> and I did not have the time today to look into the rather more
>>> complex code today.
>>> I hope this helps improve this for the next release, maybe you
>>> want to test on bare metal, too.
>>
>> Alright to close the loop with this, this slipped through the cracks because I was doing a lot of performance related work, and specifically had been testing with these patches on top of everything
>>
>> https://lore.kernel.org/linux-btrfs/cover.1602249928.git.josef@toxicpanda.com/
>>
>> These patches bring the performance up to around 40% higher than baseline
> 
> I indeed tested the linux-btrfs for-5.11 and found the performance some 50% better. I would hope that can be brought back to 5.9 levels sometime soon ;-)

Do you mean 50% better as compared to 5.9?
> 
>> .  In the meantime we'll probably push this partial revert into 5.10 stable so performance isn't sucking in the meantime.  Thanks,
> 
> That certainly makes sense for the LTS kernel series.
> 
> Thanks for looking into this,
> Merry Christmas,
> 	René Rebe
>