rust multithreading at the cache line
rust makes threads safe.
the cpu still makes them pay rent.
that is the part people miss. `Send`, `Sync`, `Arc`, `Mutex`, `RwLock`, atomics, channels, Rayon, Tokio — all of that is rust's language for describing who is allowed to touch memory. below it, the processor only sees cores, cache lines, store buffers, fences, scheduler interrupts, and coherence traffic.
the question is never: can i spawn threads?
the question is: what memory are they going to fight over?
the speedup floor.