[ibraheemdev]

> And futexes aren’t the only way to get there. Alternatives:

> - thin locks (what JVMs use)

> - ParkingLot (a futex-like primitive that works entirely in userland and doesn’t require that the OS have futexes)

Worth nothing that somewhere under the hood, any modern lock is going to be using a futex (if supported). futex is the most efficient way to park on Linux, so you even want to be using it on the slow path. Your language's thread.park() primitive is almost certainly using a futex.

[pizlonator]

ParkingLot just uses pthread mutex and cond.

Sure that uses futex under the hood, but the point is, you use futexes on Linux because that’s just what Linux gives you

[ibraheemdev]

> ParkingLot just uses pthread mutex and cond.

That's interesting, I'm more familiar with the Rust parking-lot implementation, which uses futex on Linux [0].

> Sure that uses futex under the hood, but the point is, you use futexes on Linux because that’s just what Linux gives you

It's a little more than that though, using a pthread_mutex or even thread.park() on the slow path is less efficient than using a futex directly. A futex lets you manage the atomic condition yourself, while generic parking utilities encode that state internally. A mutex implementation generally already has a built-in atomic condition with simpler state transitions for each thread in the queue, and so can avoid the additional overhead by making the futex call directly.

[0]: https://github.com/Amanieu/parking_lot/blob/739d370a809878e4...

[pizlonator]

> It's a little more than that though, using a pthread_mutex or even thread.park() on the slow path is less efficient than using a futex directly.

No, it absolutely isn’t.

The dominant cost of parking is whatever happens in the kernel and at the microarchitectural level when your thread goes to sleep. That cost is so dominant that whether you park with a futex wait or with a condition variables doesn’t matter at all.

(Source: I’ve done that experiment to death as a lock implementer back when I maintained Jikes RVM’s thin locks and then again when I wrote and maintained ParkingLot.)