Naturally, we've experimented a lot measuring context-switch cost. While pretty bad on OS X, on Linux it's quite good in many circumstances (sleep, IO interrupt) but not so good when a thread waits to be woken up by another application thread (as opposed to being woken up by some kernel event). Aside from consuming less memory, Quasar fibers shine exactly in that last scenario: when fibers wait on other fibers (they could be waiting for an actor or channel message, a condition variable, or an in-memory DB transaction). Fibers use Doug Lea's awesome fork/join thread pool for scheduling.

I hope to write about our context-switch results some day.