I'm mostly thinking Tornado/Python, where the async stuff happened via generators (IE, the "yield" keyword). But that meant there were large chunks of the pytho

I'm mostly thinking Tornado/Python, where the async stuff happened via generators (IE, the "yield" keyword). But that meant there were large chunks of the python standard library that were basically off limits because they blocked and couldn't be used with a generator, so if you used those functions the main event loop would be stuck waiting. For node, we happen to have a server that calls into a geometric modeler (for collaborative 3d modeling). Since it's doing a lot of math, you could totally conceive that while an expensive modeling operation is running and chewing through CPU cycles, all the other sessions on the system are just waiting. That's kind of a specific use case admittedly, but with threads it wouldn't even be an issue, but with async it's a problem. I get there's ways around it (offload the work to a worker process asynchronously, for instance, which is what we're doing), but it's annoying that it's a thing I have to think about when the functionality is built into the OS.