[zbentley]

Eh, not really. Async (in this semantic context) is generally about cooperative concurrency and also often about concurrent or multiplexed I/O. Pthreads aren't async by those definitions, though you can run async code within a given pthread as usual.

Goroutines are an unusual case, in that they don't have cooperative concurrency--they're pre-emptive--but the Go runtime does perform I/O using concurrent multiplexers under the hood.

So goroutines are kind of both: computation execution and code semantics look like pthreads, but I/O operations look like NodeJS on the backend.

Now, I'm not sure what "async runtime" means in the GP. If they're referring to I/O multiplexers, then they should say that. If they're referring to something else, then I'm not familiar with other uses of that term that would accurately apply to Golang.

[ori_b]

Well, that's exactly what the kernel is doing when it swaps threads. When you block on I/O, you're voluntarily pausing your thread and doing concurrent I/O with another thread.

Async and threads are a lot closer than most people think. An OS is mainly a queue for swapping between async operations, and a collection of abstracted services that the async operations can request, like network or disk i/o.