[pron]

"Plain" OS threads certainly have their place and the OS does a fine job scheduling them. It's just that the more information you have about the threads' behavior the better they can be scheduled (you can reduce latencies by keeping related fibers on the same core to share cache; that's one of the things Quasar does).

So the increased latency of the OS scheduler has little to do with the number of layers between your application and the hardware, and a lot to do about assumptions the OS can make about your code.

You most certainly want a general-purpose OS scheduler, it's just that many applications can benefit from user-level lightweight threads.