> But coroutines _are_ an option for parallelism, and an especially effective one in the I/O-bound world. Parallelism means that code executes simultaneously (

> But coroutines _are_ an option for parallelism, and an especially effective one in the I/O-bound world. Parallelism means that code executes simultaneously (on different CPUs). Coroutines (on a single threaded executor) don't do that. > The parallelism comes from the OS handling I/O scheduling for the coroutines instead of the application code. Where exactly is the parallelism that is enabled by coroutines? Coroutines are nothing more than resumable functions. When it comes to I/O performance, threads vs. async is really a false dichotomy. What we should be comparing is 1. threads + blocking I/O 2. non-blocking I/O (select, poll, epoll) 3. asynchronous I/O (io_ring on Linux, IOCP on Windows) Coroutines may only provide syntactic sugar for 2. and 3.