> Indeed, and also what about the effect of multiple cores? You could have 7 cores working on game logic and one doing GC.

Note that those independent cores all share common resources. L3 cache & memory bandwidth are both shared, and a GC workload slams both of those resources pretty heavily. So it's still going to have an impact even though it's running on its own core.

That sounds like a massive synchronization nightmare and performance overhead that'll probably not be worth it.

not sure that cpu affinity is trivially solveable to avoid mutex contention here, since game entities can often mutate one another when they interact. how do you determine which entities are computed by which cores to minimize the cost of synchronizing the work between the cores?

One strategy might be to correlate CPU affinity with spatial proximity. That is: two entities that are close together are more likely to mutate one another than two entities far apart, so the two close together should be on the same CPU/core/thread and the other should be on a different thread (probably with other entities closer to it).