We've used fork_union in spatgris (https://github.com/GRIS-UdeM/SpatGRIS) recently and got a nice speedup! That said there were some trouble with the busy wait eating 100% of CPU
I completely agree that tuning is needed for better CPU sleep scheduling. I’m hoping to look into it this October, ideally on some Alder Lake CPUs with mixed Performance/Efficiency cores and NUMA enabled.