2 ms·

And what, unconditionally futex wakes everyone? That'll be hella slow

• wbl 2y ago
  You did say uncontended!
  • pizlonator 2y ago
    So you want uncontended unlocks to make a syscall to wake the threads that aren’t there? That syscall will be worse than a CAS just because it’s a syscall. Not to mention it’ll have to lock some kernel data structure just to figure out that there aren’t any threads to wake.
    • wbl 2y ago
      I think I'm not doing a great job of explaining it. There are three states of the mutex: locked=1, unlocked=0, and sleeping=2. A thread comes along to a locked mutex, and atomically moves it to sleeping and sleeps via futex. The freeing thread unconditionally writes 0 and atomically sees the old value: if 2 wake all, if 1 no need to wake anybody. Maybe that is equally expensive to a CAS, but I don't think it ultimately is.
      • pizlonator 2y ago
        “Atomically sees the old value” means you’re doing an atomic RMW, which is more expensive than just a store with a release fence. Atomic RMW is almost as expensive as CAS. Exactly as expensive as CAS on some cpus.