Re: REGRESSION BISECTED mm, mmap: limit THP alignment of anonymous mappings to PMD-aligned sizes

[Vlastimil Babka]

On 2/4/25 01:56, Doug Smythies wrote:
> Hello,
> 
> Note: The CC list is a guess, and I am not on the two vger.kernel.org lists.
> 
> After observing a 30% reduction in the ebizzy benchmark performance, I bisected the kernel and got:
> 
> doug@s19:~/kernel/linux$ git bisect bad
> d4148aeab412432bf928f311eca8a2ba52bb05df is the first bad commit
> commit d4148aeab412432bf928f311eca8a2ba52bb05df
> Author: Vlastimil Babka <vbabka@xxxxxxx>
> Date:   Thu Oct 24 17:12:29 2024 +0200
> 
>     mm, mmap: limit THP alignment of anonymous mappings to PMD-aligned sizes

Thanks for the report. That commit fixed regressions of other workloads, but
since it's tweaking a heuristic related to THPs that can have both positive
and negative consequences, it's not surprising to see that another workload
can regress.

Since that commit is implicated, there should have been a matching
improvement for this workload in 6.7 thanks to commit efa7df3e3bb5 ?

I guess one option to proceed would be to check what kind of mappings ebizzy
creates for its performance sensitive operations, and see if they e.g. were
backed by THPs before d4148aeab41 and now they aren't. Maybe some simple
adjustment to ebizzy's allocations is possible to achieve the better
performance always and not rely on this particular heuristic.

> As a double check I reverted the commit, on top of kernel 6.14-rc1.
> I had to manually revert it, due to other changes since then.
> The previous performance of the benchmark was restored.
> 
> I actually use the sleeping-ebizzy benchmark [1].
> I use it for idle governor testing because it has yielded interesting results in the past.
> And sweep over a range of sleep times. Example graphs attached.
> 
> Legend (regression average is over interval range from 400 to 3600 uSec):
> 
> teo611: kernel 6.11, teo idle governor. Reference.
> teo614: kernel 6.14-rc1, teo idle governor. Regression 25.7%
> teo612: kernel 6.12, teo idle governor. Regression 24.4%
> teo613: kernel 6.13, teo idle governor. Regression 25.1%
> teo614-revert: kernel 6.14-rc1, with this patch reverted, teo idle governor. No regression, 1.4%
> menu: kernel 6.14-rc1, menu idle governor. Regression 26.4% 
> 
> Example command:
> ./ebizzy -m -S 20 -t 128 -a 1 -i 400
>  
> My processor: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz
> Distro: Ubuntu 24.04, server, no desktop GUI.
> 
> [1] https://github.com/pratiksampat/sleeping-ebizzy
> 
> Doug Smythies
> 
> 
> interval-sweep.png
> 
> 
> relative-performance.png
>