Async vs Threads

Start with the idea

A thread costs a stack and a scheduler slot, which is fine for a few hundred and painful for a hundred thousand. Async tasks are state machines sharing a small pool of threads, so waiting on ten thousand sockets is cheap — but a task that computes without yielding blocks everything sharing its thread. The dividing line is whether the work waits or works.

A thread is an operating-system execution context; an async task is a future that an executor polls. The async task can yield during nonblocking waits, allowing one thread to serve many waiting operations. A thread can execute CPU work in parallel with other threads when cores are available. These distinctions are more useful than saying one technology is for all I/O and the other for all CPU.