Default to single threaded actions

[blackliner]

We have a decently sized rust codebase (800k LOC), and we regularly suffer from the fact that rust actions spawn up to N parallel threads. It leads to CPU starvation (slow, but not ciritcal) and OOM events (very bad!!).

I think there are two approaches

1. make bazel aware of the "size" of an action, see Cooperative Parallelism bazel#10443. Likely a bigger effort, but eventually the right approach IMHO
2. default to single threaded executions for compilation (codegen-units) and tests (test-threads)

I see the second approach being used in this repo, but from the surrounding comments I deduce this is done because of better optimized build results.

References:

• Bazel high level issue: Cooperative Parallelism bazel#10443
• Other languages have similar issues, see for example the bazel + Nix rules, where the number of parallel jobs can go N² (blowing up your memory)

[UebelAndre]

This might be relevant to #3008

[UebelAndre]

I'd be happy to review a change to add representative resource tracking to Rustc actions. I just haven't had the time to track down if actions by default are trying to take 16 CPU. If so, we should make a global flag in Bazel that explicitly controls that and then ensure all actions use that.

[blackliner]

with #3112 merged, is this problem "solved"?

[hofbi]

@UebelAndre @illicitonion I think there is a similar issue for tests. According to https://doc.rust-lang.org/rustc/tests/index.html#--test-threads-num_threads

By default, uses the amount of concurrency available on the hardware as indicated by available_parallelism.

So I wonder if it would make sense to set --test-threads=1 to make sure bazel is in control of all parallel actions.