[the8472]

I have played around a tiny bit with par_iter over blocking IO tasks and seen some sched_yield() loops burning CPU time instead of backing off to futex_wait. That seems suboptimal and not exactly "the best ever" I'd expect from a parallelism library.

[pcwalton]

They should be doing that for a few iterations before backing off. Otherwise you end up with bad scheduling leading to slow warmups, among other problems.

You shouldn't use rayon for blocking I/O; that's not what it's designed for. Rayon is a parallelism library, not a concurrency library.