The biggest thing when making a CI/CD fast is minimizing the work that you need to do each time. If you’re having to compile all the dependencies from scratch every time, the biggest win will come from finding a way to cache that, regardless of what CPUs are available. We have a large project, but the actual compilation step takes < 10 seconds in our CI unless we are changing a dependency.

Tiered caches will help with the build time a lot. On CircleCI, even with small test runners, you are able to cache the hex deps, the compiled code, and PLT files if dialyzer is your thing.

I do this for all my applications, but here is an example for an open source project:

Fair enough! The Elixir compiler is parallel, so you should be able to benefit from multiple cores. SSD speed is of course critical. 64gb of ram is wildly more than enough.

To follow up, on a Xeon 2150B (10/20 cores):

$ time mix compile --force
Compiling 418 files (.ex)
Generated <our_company_app> app
mix compile --force  76.51s user 16.60s system 664% cpu 14.020 total

To be honest, I expected the Elixir compiler to be able to utilise all cores.

Still pretty fast though.

Yep, I’ve heard the dependency graph can severely impact the parallelisation of compilation
and it makes sense.