Re: Parallel downloads

["Ludovic Courtès" <ludo@gnu.org>]

Hi,

Leo Famulari <leo@famulari.name> skribis:

> On Sat, Nov 09, 2019 at 06:40:56PM +0100, Ludovic Courtès wrote:
>> Like I wrote, it’s not that simple (we’d first need the daemon to
>> distinguish substitution jobs from other jobs, but note that there are
>> also “downloads” that are actually derivation builds), and it’s not
>> clear to me that it’s overall beneficial anyway: it’s not supposed to be
>> faster to download 10 things in parallel from ci.guix.gnu.org, than to
>> download them sequentially.
>
> Parallel downloading is not faster in terms of overall transfer rate
> from ci.guix.gnu.org.
>
> However, installing things with Guix involves downloading a lot of very
> small files like derivations, and Guix spends a lot of time initiating
> these downloads.

Good point.

Note that .drv files are never downloaded, but nevertheless it’s true
that there are often small files like the “module-import-compiled”
things.  This happens when building a system, but not so much when
building a package, though.

> For example, I can download things at 100 megabits, but when Guix needs
> to sequentially download 50 10-kilobyte files, it may take an entire
> minute.
>
> So there is a huge speedup with parallel downloading.

One thing I’d like to get rid of is the initial HTTP GET for
/nix-cache-info which is completely useless now.  That could help a bit.

Thanks,
Ludo’.