[dunfell] ref-manual: Add XZ_THREADS and XZ_MEMLIMIT

From: Richard Purdie <richard.purdie@linuxfoundation.org>

XZ_THREADS and XZ_MEMLIMIT were introduced in dunfell.

[RP improved an original patch from Paul]
Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
---
 documentation/ref-manual/ref-variables.rst | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

Hi Quentin

Thanks for the patch!

On 7/18/22 18:17, Quentin Schulz wrote:
> From: Richard Purdie <richard.purdie@linuxfoundation.org>
>
> XZ_THREADS and XZ_MEMLIMIT were introduced in dunfell.
>
> [RP improved an original patch from Paul]
> Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
> Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
> ---
>   documentation/ref-manual/ref-variables.rst | 21 ++++++++++++++++++++-
>   1 file changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/documentation/ref-manual/ref-variables.rst b/documentation/ref-manual/ref-variables.rst
> index 49e756cfb..c5facfa98 100644
> --- a/documentation/ref-manual/ref-variables.rst
> +++ b/documentation/ref-manual/ref-variables.rst
> @@ -8745,4 +8745,23 @@ system and gives an overview of their function and contents.
>   
>         The default value of ``XSERVER``, if not specified in the machine
>         configuration, is "xserver-xorg xf86-video-fbdev xf86-input-evdev".
> -
> +
> +   :term:`XZ_THREADS`
> +      Specifies the number of parallel threads that should be used when
> +      using xz compression.
> +
> +      By default this scales with core count, but is never set less than 2
> +      to ensure that multi-threaded mode is always used so that the output
> +      file contents are deterministic. Builds will work with a value of 1
> +      but the output will differ compared to the output from the compression
> +      generated when more than one thread is used.
> +
> +      On systems where many tasks run in parallel, setting a limit to this
> +      can be helpful in controlling system resource usage.
> +
> +    :term:`XZ_MEMLIMIT`
> +      Specifies the maximum memory the xz compression should use as a percentage
> +      of system memory. If unconstrained the xz compressor can use large amounts of
> +      memory and become problematic with parallelism elsewhere in the build.
> +      "50%" has been found to be a good value.
> +


Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
and merged into the "dunfell" branch.

Thanks again!
Michael.