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

[Josef Bacik <josef@toxicpanda.com>]

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.  In the meantime we'll probably push this partial revert into 
5.10 stable so performance isn't sucking in the meantime.  Thanks,

Josef