> Out of interest what were they?

The one that immediately comes to mind is cvc5... not super recently though.

I suspect that "tried to" is doing a bit of work here. The fact that it was failing and swapping out probably meant that the more memory heavy g++ processes were going slower than the memory light ones, resulting in more of them running simultaneously than would likely have happened in a normal successful build. Still, this was on a system with 32GB of ram, so it was using roughly that before swapping would slow down more memory intensive processes.