If you hate garbage collection pauses (which most Rust users do) then don't use async.
Sure, if you need to run 20k connections then use async. But the fact of the matter is that the vast majority of software is not going to take on 20k connections. Those people (i.e the majority of software devs) should use threads, because they are much easier to reason about and work with.
It's just doing a loop and a call to poll(), that's it.
It's way way way less expensive than using threads. Of course you must give control to your main loop every once in a while, so if you have a long computation you either create a thread or split it and return control to your main loop.
It's how all GUI programming has always been done.
> Whereas async simply locks the CPUWhereas async simply locks the CPU
This is also completely nonsense, context switching behavior is OS dependent and your average general purpose kernel is not cooperative. You will run for your allotted quanta or reschedule when you run out of coroutines that can execute without waiting for resources.
True, but if all you are using is async, then you're basically back at Windows 3.1 cooperative multitasking, except now within a Rust program.
As other commenters have pointed out, cooperative multitasking is actually a great fit for I/O bound code.
For other things, async task (or green thread, whatevs) per connection is a very nice model that you can't do with thread per connection because I don't think OSes are happy to gave hundreds of thousands of threads.
I don’t think I ever seem this patterns, What I’ve seen is either a thread pool for tasks (what essentially async is), or dedicated threads for each parts of the processing (a thread for the UI, and a thread for some background services like playing music).
this doesn't really contradict with
> async task (or green thread, whatevs) per connection
The async tasks will probably run on a thread pool (e.g. in tokio).