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

[Julien Lepiller <julien@lepiller.eu>]

Le Thu, 25 Nov 2021 13:53:25 +0100,
Ludovic Courtès <ludo@gnu.org> a écrit :

> Hello,
> 
> Julien Lepiller <julien@lepiller.eu> skribis:
> 
> > After so long, I managed to find the time to go over the comments
> > and improve my patches. I tested the new daemon for a bit, and it's
> > working as expected so far :D  
> 
> On a recent daemon, have you seen cases where having multiple
> downloads in parallel speeds things up?
> 
> The analysis in
> <https://guix.gnu.org/en/blog/2021/getting-bytes-to-disk-more-quickly/>
> suggests that at the time you first submitted this patch, substitution
> speed (which is different from raw download speed) was often
> CPU-bound. This is no longer the case, meaning that downloads should
> now be network-bound or almost.
> 
> Thanks,
> Ludo’.

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.

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 :)

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.