[arch-general] Multithreaded XZ

[Christian Hesse]

Jan Alexander Steffens <jan.steffens at gmail.com> on Mon, 2014/12/22 04:08:
> On Mon, Dec 22, 2014 at 3:25 AM, Mark Lee <mark at markelee.com> wrote:
> > Now that xz is multithreaded (not available in Arch Repos yet); I was
> > wondering if package building and installing would, by default,
> > utilize multiple threads.

Sure. I do use the multi-threaded version since the end of 2011 with my
xz-git package [0] from AUR.
Worked perfectly since then.

> It won't, since the output of XZ in multi-threaded mode is different
> from single-threaded mode. This will break anything relying on getting
> repeatable output from compression.

Yes, the output is different (and usually a little larger) for streams larger
than block-size. But that does not matter. Every xz version can decompress
every xz-compressed stream - including those created with multi-threaded xz.

For Arch packages there is nothing that requires repeatable output.
Building a package will generate new output one way or another - even a
changed package file timestamp will change the output.
Or did I get you wrong here?

> I'm not even sure it's possible to
> run XZ in multi-threaded mode with a single thread. xz -T1 uses
> single-threaded mode. xz -T1 --block-size=24M is also different from
> xz -T2 --block-size=24M.
>
> Multi-threaded decompression also requires a stream created using
> multi-threaded mode.

Yes. But it falls back to single-threaded mode without complaints.

[0] https://aur.archlinux.org/packages/xz-git/
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