Function rayon::join

pub fn join<A, B, RA, RB>(oper_a: A, oper_b: B) -> (RA, RB) where
    A: FnOnce() -> RA + Send,
    B: FnOnce() -> RB + Send,
    RA: Send,
    RB: Send, 

The join function takes two closures and potentially runs them in parallel. It returns a pair of the results from those closures.

Conceptually, calling join() is similar to spawning two threads, one executing each of the two closures. However, the implementation is quite different and incurs very low overhead. The underlying technique is called "work stealing": the Rayon runtime uses a fixed pool of worker threads and attempts to only execute code in parallel when there are idle CPUs to handle it.

Warning about blocking I/O

The assumption is that the closures given to join() are CPU-bound tasks that to do not perform I/O or other blocking operations. If you do perform I/O, and that I/O should block (e.g., waiting for a network request), the overall performance may be poor. Moreover, if you cause one closure to be blocked waiting on another (for example, using a channel), that could lead to a deadlock.

Panics

No matter what happens, both closures will always be executed. If a single closure panics, whether it be the first or second closure, that panic will be propagated and hence join() will panic with the same panic value. If both closures panic, join() will panic with the panic value from the first closure.