That's a tricky question. My understanding is that while it's (in theory, modulo compiler bugs and features) a 0-cost abstraction over different underlying syst

That's a tricky question. My understanding is that while it's (in theory, modulo compiler bugs and features) a 0-cost abstraction over different underlying system APIs, those different underlying system APIs aren't necessarily the same cost. For example, if I'm trying to read from a socket in the synchronous world, I just issue the `read` system call. But in the async world I'm going to do quite a bit more: - Create an epoll descriptor. - Add my socket to that descriptor. - Poll the descriptor for a readiness notification. - Read the descriptor. Those first three system calls weren't required in the synchronous version, and unless the read is large enough to overshadow them, they represent some additional cost. But that cost is required by the OS itself, not by Rust's abstractions. Someone with more experience writing Mio code might want to jump in and correct me here though.