Changing kernel configuration options related to the SquashFS decompression

[Bohdan Khomutskyi]

Hello,

While working on the change proposal 
https://fedoraproject.org/wiki/Changes/OptimizeSquashFS

I discovered that several kernel configuration options are set 
sub-optimally in Fedora. In RHEL 8, the optimal configuration is used.
If we set those options to match RHEL 8, a performance improvement could 
be achieved while reading the SquashFS filesystem.

Below you can see an excerpt from the configuration help:

Set in RHEL 8:

SQUASHFS_FILE_DIRECT
       Directly decompress file data into the page cache.
       Doing so can significantly improve performance because
       it eliminates a memcpy and it also removes the lock contention
       on the single buffer.

SQUASHFS_DECOMP_MULTI_PERCPU
       By default Squashfs uses a single decompressor but it gives
       poor performance on parallel I/O workloads when using multiple CPU
       machines due to waiting on decompressor availability.

This decompressor implementation uses a maximum of one
       decompressor per core.  It uses percpu variables to ensure
       decompression is load-balanced across the cores.

Set in Fedora:
SQUASHFS_DECOMP_SINGLE
       Traditionally Squashfs has used single-threaded decompression.
       Only one block (data or metadata) can be decompressed at any
       one time.  This limits CPU and memory usage to a minimum.

SQUASHFS_FILE_CACHE
       Decompress file data into an intermediate buffer and then
       memcopy it into the page cache.

The SquashFS filesystem is used on installation medium. So making the 
change I propose should improve the user experience during installation. 
It should also improve the user experience while booting from Fedora 
Live image.

Kind regards,

[Don Zickus]

On Fri, Sep 18, 2020 at 10:49:09AM +0200, Bohdan Khomutskyi wrote:
> Hello,
>
> While working on the change proposal
> https://fedoraproject.org/wiki/Changes/OptimizeSquashFS
>
> I discovered that several kernel configuration options are set sub-optimally
> in Fedora. In RHEL 8, the optimal configuration is used.
> If we set those options to match RHEL 8, a performance improvement could be
> achieved while reading the SquashFS filesystem.
>
> Below you can see an excerpt from the configuration help:

Would you be willing to submit a merge request with these changes for
Fedora?  Then we can have a discussion and review.

Cheers,
Don
> Set in RHEL 8:
>
> SQUASHFS_FILE_DIRECT
> Â Â Â  Â  Directly decompress file data into the page cache.
> Â Â Â  Â  Doing so can significantly improve performance because
> Â Â Â  Â  it eliminates a memcpy and it also removes the lock contention
> Â Â Â  Â  on the single buffer.
>
> SQUASHFS_DECOMP_MULTI_PERCPU
> Â Â Â  Â  By default Squashfs uses a single decompressor but it gives
> Â Â Â  Â  poor performance on parallel I/O workloads when using multiple CPU
> Â Â Â  Â  machines due to waiting on decompressor availability.
>
> Â Â Â  Â  This decompressor implementation uses a maximum of one
>       decompressor per core.  It uses percpu variables to ensure
> Â Â Â  Â  decompression is load-balanced across the cores.
>
> Set in Fedora:
> SQUASHFS_DECOMP_SINGLE
> Â Â Â  Â  Traditionally Squashfs has used single-threaded decompression.
> Â Â Â  Â  Only one block (data or metadata) can be decompressed at any
>       one time.  This limits CPU and memory usage to a minimum.
>
> SQUASHFS_FILE_CACHE
> Â Â Â  Â  Decompress file data into an intermediate buffer and then
> Â Â Â  Â  memcopy it into the page cache.
>
> The SquashFS filesystem is used on installation medium. So making the change
> I propose should improve the user experience during installation. It should
> also improve the user experience while booting from Fedora Live image.
>
> Kind regards,
>
> -- 
> Bohdan Khomutskyi
> Software engineer
> Red Hat
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