Re: Questions and Notes regarding Offloading and Substitutes

[Ludovic Courtès]

Hi!

Sorry for the delay, that’s what you get for writing long messages.  ;-)

Carlos Sosa <address@hidden> skribis:

> What is a little bit confusing is how `guix build` and offloading works.
> To be honest, in the begining, offloading was not that straight forward
> since the manual lacks to point out that `guix offloading` is using
> `lsh`. So there are certain steps that need to be addressed like,
> creating a 'lsh' key and then exporting it to an openssh public key.
> After that you might have a problem with the cyphers available in
> OpenSSH and lsh, which are disabled by default in OpenSSH. After setting
> all of that, builds offloading works flawlessly, I was really happy I
> haven't encountered any problem.

I must say that the fact that lsh was not mentioned was originally
intentional, on the grounds that the implementation would “soon” switch
to Guile-SSH instead (which is compatible with OpenSSH, not lsh.)  But
that hasn’t happened yet, although there’s a branch, wip-guile-ssh.

> Now what I don't get is why offloading mostly works for builds and not
> for guix package installation. I might be confusing what builds are for,
> but shouldn't an already finished build be used by `guix package` when
> installing a package. For example, if I do 'guix build emacs' that will
> build emacs and its dependencies, and I can call later `guix package -i
> emacs`, right? When I do `guix package -i emacs` with the daemon with
> '--max-jobs=0' instead of offloading the compilation and built to any of
> the machines in 'machines.scm', it seems to offload some bits but still
> compile some components in the machine. I think the latter is fine, but
> I consider it's best if they did everything. In my case, I own two small
> netbooks that I would love to offload all of the process of building and
> compiling to a bigger node capable of doing that, but still keeping a
> store and profile in those machines.

Normally everything gets offloaded provided there are available slots on
the target machines (as per the ‘parallel-builds’ field.)

Now, some derivations explicitly ask not to be offloaded (search for
#:local-build? in the code.)  This is typically derivations that do very
little work and produce small files, where the overhead of offloading
would outweigh the gain.

For example, the derivation that builds a profile (when running ‘guix
package -i’) is not subject to offloading.  This may be what you were
observing no?

> I'm having a couple of problems with substitutes, I can't seem to be
> able to send all of the needed substitutes, for let us say the emacs
> package, to the small netbooks from the big node at home. They do grab
> some derivations and other elements, but seem to give me a Nix failure
> from time to time. Are substitutes what I'm ultimately looking for,
> having a build node build everything and send it as a substitute to the
> smaller nodes?

What do you mean by “send all of the needed substitutes”?  Note that
offloading and substitutes are two different mechanisms, even though
they seem similar.

HTH!

Ludo’.