Correction on futexes: You've mostly got hot and cold inverted, here. A hot mutex/futex or code segment is one that many threads are trying to use at once. A co

Correction on futexes: You've mostly got hot and cold inverted, here. A hot mutex/futex or code segment is one that many threads are trying to use at once. A cold one is one that is not very busy. So cold futexes are typically purely userspace and hot ones become less so.