In theory async/await can be compiled to something as efficient as if you wrote all the non-blocking logic by hand so yeah, it's a "zero cost" abstraction. For

In theory async/await can be compiled to something as efficient as if you wrote all the non-blocking logic by hand so yeah, it's a "zero cost" abstraction. For green threads the stdlib would have to be designed around them (again) or they'd feel like something bolted on the side with two sets of APIs for everything. This is also the case for async/await but that means green threads don't have an advantage there. If you have to worry about Task vs Thread, make sure you all the right APIs from each, figure out what to do for TLS in Tasks, etc then you may as well just use the one that is more efficient.