My current workflow has a lot of "Run make -j<lots> to build, followed by parallel -j<lots> to run all the tests", but sometimes I want to compare/test multiple different versions of the code (in a way which, sadly, doesn't work well with incremental builds). In that case it'd be nice to be able to just spin everything up and not have to worry about overloading/under-loading the machine I'm working on.

I know what you mean about the load average limiter - parallel behaves like that too. I think the --delay option to parallel is supposed to solve that (I haven't tried it - will try tomorrow), but I don't know if make has anything similar.

Finally, on further reading, it definitely seems technically possible, even if it hasn't been done so far. The make documentation has a section on the jobserver protocol, which looks complete enough to write both the client and server parts: https://www.gnu.org/software/make/manual/html_node/Job-Slots...

So if nothing exists so far, it's something I might look into writing myself.