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

["René Rebe" <rene@exactcode.de>]

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 ;-)

> .  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

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