> Cloud computing (i.e. datacenter software) requires zillions of concurrent "fibers" on a much smaller number of OS threads.

As a guy who actually wrote "datacenter software" for over a decade: you don't speak from experience and what you said is false.

Basically, OS threads are the smallest and lightest form of concurrency currently in existence. Any userspace 'fiber' will be heavier and slower than actually spawning a pthread.

The problem is when you couple pthreads with interpreter runtimes. (Python, Lua, Ruby, etc.) These runtimes (and especially the GC engines they use!) don't play nice with OS threads, so you're forced to invent various rube-goldberg fiber-like contraptions on async primities.

The end result is slower, heavier and more brittle than any pthread-only solution, but interpreter runtimes necessarily impose overhead, you learn to deal with it.