Re: When substitute download + decompression is CPU-bound

["Ludovic Courtès" <ludo@gnu.org>]

Hi Pierre,

Pierre Neidhardt <mail@ambrevar.xyz> skribis:

> Ludovic Courtès <ludo@gnu.org> writes:
>
>> Well, ‘guix publish’ would first need to create multi-member archives,
>> right?
>
> Correct, but it's trivial once the bindings have been implemented.

OK.

>> Also, lzlib (which is what we use) does not implement parallel
>> decompression, AIUI.
>
> Yes it does, multi-member archives is a non-optional part of the Lzip
> specs, and lzlib implemetns all the specs.

Nice.

>> Even if it did, would we be able to take advantage of it?  Currently
>> ‘restore-file’ expects to read an archive stream sequentially.
>
> Yes it works, I just tried this:
>
> cat big-file.lz | plzip -d -o big-file -
>
> Decompression happens in parallel.
>
>> Even if I’m wrong :-), decompression speed would at best be doubled on
>> multi-core machines (wouldn’t help much on low-end ARM devices), and
>> that’s very little compared to the decompression speed achieved by zstd.
>
> Why doubled?  If the archive has more than CORE-NUMBER segments, then
> the decompression duration can be divided by CORE-NUMBER.

My laptop has 4 cores, so at best I’d get a 4x speedup, compared to the
10x speedup with zstd that also comes with much lower resource usage,
etc.

> All that said, I think we should have both:
>
> - Parallel lzip support is the easiest to add at this point.
>   It's the best option for people with low bandwidth.  This can benefit
>   most of the planet I suppose.
>
> - zstd is best for users with high bandwidth (or with slow hardware).
>   We need to write the necessary bindings though, so it will take a bit
>   more time.
>
> Then the users can choose which compression they prefer, mostly
> depending on their hardware and bandwidth.

Would you like to give parallel lzip a try?

Thanks!

Ludo’.