[bug#39728] [PATCH v2] Allow parallel downloads and builds

[Ludovic Courtès]

Hi!

Julien Lepiller <julien@lepiller.eu> skribis:

> I would still say yes, because the output from berlin is often much
> less than my throughput. With multiple downloads in parallel it at
> least feels quicker, probably because I can download at full speed.

It would be nice to measure that because like I wrote, I think we’re
pretty much network-bound these days, at least with zstd and
uncompressed downloads.

> In any case, I see often a build start while downloads are in progress,
> so I think it's still a win if you can get a few derivations built
> while waiting for a big download to finish at the same time :)

True!  Overlapping downloads and builds sounds like a good idea.

> At some point we might want to prioritize builds/downloads that help
> unlock as much builds as possible early, so we don't have builds
> waiting for downloads.

Right now the daemon starts with substitutes and builds afterwards.

BTW, we’re assuming downloads = substitutes in this whole discussion,
but we could/should take fixed-output derivations into account too.

I’ll take a closer look later on…

Thanks,
Ludo’.