[bug#33643] [PATCH] gnu-build-system: Enable xz to decompress in parallel.

[Christopher Baines <mail@cbaines.net>]

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Leo Famulari <leo@famulari.name> writes:

> On Thu, Dec 06, 2018 at 07:56:15AM +0000, Christopher Baines wrote:
>> It can take a little while to decompress some packages with large xz
>> compressed source tar files. xz includes support for parallelism, so enable
>> this using the parallel job count for the overall derivation.
>
> The xz man page says that multi-threaded decompression isn't implemented
> yet, unfortunately.

Ah, interesting. Having a read myself now, it also says it:

  "will work on files that contain multiple blocks with size information
   in block headers.  All files compressed in multi-threaded mode meet
   this condition, but files compressed in single- threaded mode don't
   even if --block-size=size is used."

So, if -T was used to compress the data, then it sounds like it'll work
to decompress it. I guess this adds a little more uncertainty to the
benefit of this change, as the impact is dependent on the way the source
data is compressed.

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