Re: Possible performance regression with CONFIG_SQUASHFS_DECOMP_SINGLE

[Phillip Lougher]

Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> wrote:

> Hello,
> 
> This an issue we had with v5.15 that we have since successfully worked around.
> I am reporting it here as a pointer in case someone else runs into this and as
> a heads up that there seems to be an underlying performance regression, so
> here it goes:
> 

[snip]

> 
> Looking through the original kernel configuration we found that it has
>  CONFIG_SQUASHFS_DECOMP_SINGLE=y and CONFIG_SQUASHFS_FILE_CACHE=y.
> 
> Once changed to CONFIG_SQUASHFS_FILE_DIRECT=y and
> CONFIG_SQUASHFS_DECOMP_MULTI_PERCPU=y, we were below 40s as we want.

This sounds like the slow-down that was introduced by commit c1f6925e1091
"mm: put readahead pages in cache earlier" (Linux V5.8)

This commit prevents Squashfs from doing it's own readahead, which
causes a slow-down in performance.  The slow-down is noticable when
using a single decompressor (CONFIG_SQUASHFS_DECOMP_SINGLE=y), and
can be solved by moving to a multi-decompressor configuration, because
it removes contention on a single buffer in the single decompressor case.

This has been already been fixed by commit 9eec1d897139
"squashfs: provide backing_dev_info in order to disable read-ahead"
which is in Linux 5.17-rc1.

Phillip