GitHub Actions backend: Builds aren't parallelized?

[RyanGlScott]

When building dependencies in haskell-ci–generated GitHub Actions configuration, the installation completely lacks any parallelism. (Example.) I wonder if the backend needs to use -j2 somewhere, like what the Travis backend uses?

[RyanGlScott]

The Bash backend, judging from the source code, seems to suffer from a similar problem. Curiously, -j2 is used when installing doctest:

haskell-ci/src/HaskellCI/Bash.hs

Line 44 in e991d01

run_cmd_if range "$CABAL v2-install $ARG_COMPILER --ignore-project -j2 doctest --constraint='doctest ^>=0.17'"

But nowhere else.

[phadej]

Yes. With bash it's tricky, as it depends on users machine/current use, which -j is reasonable. For GitHub Actions we should use whatever cores GHA provides. Please find the environment info, and add/change according -j.

The actual build is better to keep -j1 though. Why? Because interleaved output is hard to parse. This can be configurable, but default should be no parallelism.

[RyanGlScott]

For GitHub Actions we should use whatever cores GHA provides. Please find the environment info, and add/change according -j.

According to this, GitHub Actions virtual machines use 2-core CPUs, so -j2 seems reasonable.

The actual build is better to keep -j1 though. Why? Because interleaved output is hard to parse. This can be configurable, but default should be no parallelism.

Absolutely. To be clear, when I say "parallelism", I'm exclusively referring to installation of dependencies, not building the main packages themselves.