Actually it can work quite well without language-level support. Python's gevent library [0] shows that. It "simply" monkey-patches the standard library's IO fun

Actually it can work quite well without language-level support. Python's gevent library [0] shows that. It "simply" monkey-patches the standard library's IO functions to implicitly switch to another "greenlet" when blocking IO is performed. On the user side you don't really see anything of that (no "await" or similar). Admittedly, that hides possible points of context switches, but is still far less dangerous with regard to race conditions than OS threads. Unfortunately, gevent seems to be shunned quite a bit and I haven't seen other languages do similar things. [0] http://www.gevent.org/ http://www.gevent.org/