Re: [ptxdist] [PATCH] image-root-cpio.in: Use multithreaded XZ (revisited).

[Michael Olbrich <m.olbrich@pengutronix.de>]

Hi,

On Sun, Feb 25, 2024 at 03:49:06PM +0100, Christian Melki wrote:
> So, after asking questions about the default multithread mode
> of xz 5.6.0 and determinism, it turned out that there
> is a common misconception about determinism in xz.
> 
> XZ compression varies between the two compressors. Ie, the single
> thread and multi thread variant. T0 always uses the multithread
> one, even if used on one core. But T1 uses the singlethread
> compressor.
> 
> This has caused a very common misconception about determinism
> variability with number of threads. I've seen this discussion
> on a lot of places. So albeit true, it only applies to the
> distinct compressors, not how many threads are used in
> multithread mode.
> 
> Now after giving this some thought I've decided to reenable
> threaded XZ, using T0. T0 should produce the same results always,
> on any target. Even unicore machines. And with the multithread
> compressor now being the default, users should end up with the
> same results unless they are actively using T1.

So if I understand it correctly then multithreaded compression results i a
slightly larger output file. But it also means that it can be extracted
with multiple threads.
The question is: will that actually happen? The initramfs is extracted by
the kernel and I'm not sure if it supports that.

Michael

> Signed-off-by: Christian Melki <christian.melki@t2data.com>
> ---
>  platforms/image-root-cpio.in | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/platforms/image-root-cpio.in b/platforms/image-root-cpio.in
> index 0338175a0..546dda7fb 100644
> --- a/platforms/image-root-cpio.in
> +++ b/platforms/image-root-cpio.in
> @@ -84,7 +84,7 @@ config IMAGE_ROOT_CPIO_COMPRESSION_UTIL
>  	default ""			if IMAGE_ROOT_CPIO_COMPRESSION_MODE_NONE
>  	default "gzip"			if IMAGE_ROOT_CPIO_COMPRESSION_MODE_GZ
>  	default "zstd -T0 -19"		if IMAGE_ROOT_CPIO_COMPRESSION_MODE_ZSTD
> -	default "xz --check=crc32"	if IMAGE_ROOT_CPIO_COMPRESSION_MODE_XZ
> +	default "xz -T0 --check=crc32"	if IMAGE_ROOT_CPIO_COMPRESSION_MODE_XZ
>  	default "lzop"			if IMAGE_ROOT_CPIO_COMPRESSION_MODE_LZOP
>  
>  endif
> -- 
> 2.34.1
> 
> 
> 

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