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

["Doug Smythies" <dsmythies@telus.net>]

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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@suse.cz>
Date:   Thu Oct 24 17:12:29 2024 +0200

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

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


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