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Thoughts on building things for substitutes and the Guix Build Coordinat

From: Christopher Baines
Subject: Thoughts on building things for substitutes and the Guix Build Coordinator
Date: Tue, 17 Nov 2020 20:45:53 +0000
User-agent: mu4e 1.4.13; emacs 27.1


In summary, this email lists the good things and bad things that you
might experience when using the Guix Build Coordinator for providing
substitutes for Guix.

So, over the last ~7 months I've been working on the Guix Build
Coordinator [1]. I think the first email I sent about it is [2], and I'm
not sure if I've sent another one. I did prepare a talk on it though
which goes through some of the workings [3].


Over the last few weeks I've fixed up and tested the Guix services for
the Guix Build Coordinator, as well as fixing some major issues like it
segfaulting frequently.

I've been using the Guix Build Coordinator build substitutes for, which is my testing ground for providing
substitutes. I think it's working reasonably well.

I wanted to write this email though to set out more about actually using
the Guix Build Coordinator to build things for substitutes, to help
inform any conversations that happen about that.

First, the good things:

The way the Guix Build Coordinator generates compressed nars where the
agent runs, then sends them over the network to the coordinator has a
few benefits. The (sometimes expensive) work of generating the nars
takes place where the agents are, so if you've got a bunch of machines
running agents, that work is distributed.

Also, when the nars are received by the coordinator, you have exactly
what you need for serving substitutes. You just generate narinfo files,
and then place the nars + narinfos where they can be fetched. The Guix
Build Coordinator contains code to help with this.

Because you aren't copying the store items back in to a single store, or
serving substitutes from the store, you don't need to scale the store to
serve more substitutes. You've still got a bunch of nars + narinfos to
store, but I think that is an easier problem to tackle.

This isn't strictly a benefit of the Guix Build Coordinator, but in
contrast to Cuirass when run on a store which is subject to periodic
garbage collection, assuming you're pairing the Guix Build Coordinator
with the Guix Data Service to provide substitutes for the derivations,
you don't run the risk of garbage collecting the derivations prior to
building them. As I say, this isn't really a benefit of the Guix Build
Coordinator, you'd potentially have the same issue if you ran the Guix
Build Coordinator with guix publish (on a machine which GC's) to provide
derivations, but I thought I'd mention it anyway.

The Guix Build Coordinator supports prioritisation of builds. You can
assign a priority to builds, and it'll try to order builds in such a way
that the higher priority builds get processed first. If the aim is to
serve substitutes, doing some prioritisation might help building the
most fetched things first.

Another feature supported by the Guix Build Coordinator is retries. If a
build fails, the Guix Build Coordinator can automatically retry it. In a
perfect world, everything would succeed first time, but because the
world isn't perfect, there still can be intermittent build
failures. Retrying failed builds even once can help reduce the chance
that a failure leads to no substitutes for that builds as well as any
builds that depend on that output.

Now the not so good things:

The Guix Build Coordinator just builds things, if you want to build all
Guix packages, you need to work out the derivations, then submit builds
for all of them. There's a script I wrote that does this with the help
of a Guix Data Service instance, but that might not be ideal for all
deployments. Even though it can handle the building of things, and most
of the serving substitutes part (just not the serving bit), some other
component(s) are needed.

Because the build results don't end up in a store (they could, but as
set out above, not being in the store is a feature I think), you can't
use `guix gc` to get rid of old store entries/substitutes. I have some
ideas about what to implement to provide some kind of GC approach over a
bunch of nars + narinfos, but I haven't implemented anything yet.

There could be issues with the implementation… I'd like to think it's
relatively simple, but that doesn't mean there aren't issues. For some
reason or another, getting backtraces for exceptions rarely works. Most
of the time the coordinator tries to print a backtrace, the part of
Guile doing that raises an exception. I've managed to cause it to
segfault, through using SQLite incorrectly, which hasn't been obvious to
fix at least for me. Additionally, there are some places where I'm
fighting against bits of Guix, things like checking for substitutes
without caching, or substituting a derivation without starting to build

Finally, the instrumentation is somewhat reliant on Prometheus, and if
you want a pretty dashboard, then you might need Grafana too. Both of
these things aren't packaged for Guix, Prometheus might be feasible to
package within the next few months, I doubt the same is true for Grafana
(due to the use of NPM).

I think that's a somewhat objective look at what using the Guix Build
Coordinator might be like at the moment. Just let me know if you have
any thoughts or questions?



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