I think you're generally on the right track. Small correction: fibers (blocking or non-blocking) never used threads in any way. Fibers are "stackful coroutines

I think you're generally on the right track. Small correction: fibers (blocking or non-blocking) never used threads in any way. Fibers are "stackful coroutines", another concurrency primitive. They're pretty hard to understand, and unfortunately I can't find a good explanation of "Ruby Fibers" that I can link to.