best options to try to avoid bogging down user's systems with parallel builds?

[Britton Kerin]

I just made a script to pre-build all git revisions in a git-bisect,
so the actual bisection testing that requires human attention can be
done without being interrupted by a build each time.

I'd like to avoid making user system go unresponsive.  Builds require
really different memory etc. so it's hard to guess what's reasonable
here.  I'm considering this:

parallel --nice=17 --load=80%

I'd also like to address memory use, but I'm not sure how best to do it.

Does --memsuspent accept at percent or not?  It seems like something
like "don't start any more jobs and suspend some at 80% memory used"
might be a reasonable guess but I don't know how to specify it.  I'm
reluctant to use fixed quantities of memory.

I'd like to avoid --memfree because killing jobs might produce
confusion about build results, and restarting them automatically might
not work.

Any advice appreciated.

Britton