Sorry, you're right, I was thinking of write barrier schemes. Off the top of my head I think most non-moving GCs use only write barriers and not read barriers

Sorry, you're right, I was thinking of write barrier schemes. Off the top of my head I think most non-moving GCs use only write barriers and not read barriers now, though, along with writes being less common than reads, which is maybe even better than what I said. https://webkit.org/blog/7122/introducing-riptide-webkits-retreating-wavefront-concurrent-garbage-collector/ https://webkit.org/blog/7122/introducing-riptide-webkits-ret... for example only uses a write barrier, and so does Lua's tricolor GC. The closest modern analogue would maybe be something like Metronome, which is a moving GC that also advertises as real-time, but has a read barrier they intentionally tried to make very cheap and optimizable - I guess you could also coalesce Treadmill read barriers to only one mutex + color transition for multiple fields of an object all at once, though.