That's only happening for IO operations which get really blocked on syscall level, which means no async version is available. For the most important IO operatio

That's only happening for IO operations which get really blocked on syscall level, which means no async version is available. For the most important IO operations (sockets) the asynchronous variants are used internally. Which means if a read/write can not be performed immediately, the goroutine will register at the netpoller component (which is a wrapper around epoll/kqueue/iocp for IO status updates) for IO status updates, and then the goroutine will yield and wait until IO becomes possible again. The current thread can execute another goroutine until then. This means for goroutines blocked on network no extra OS threads are required. For other blocking OS operations (maybe file IO) extra OS threads are required.