> The weakness of callbacks is the lack of multicore support. You suggest that gevent won't work in preemptive environment if it wants to support multiple core

> The weakness of callbacks is the lack of multicore support. You suggest that gevent won't work in preemptive environment if it wants to support multiple cores, alas I'm still not convinced as to why. The greenlets are cooperatively scheduled coroutines, as we know, but with proper multicore support I don't see why they couldn't adapted to be preemptively scheduled. If we're imagining that Python now has multicore support, we can also imagine greenlets being updated to work with it. Assuming gevent would receive no modifications to work with such an important update to Python is why I accused you of speculating. > It's because gevent context switches on I/O events, not preemptively, so you have some control over when context switches happen. That's correct. They are cooperatively scheduled coroutines. If someone wants to context switch, they can write code that does it or wait for I/O events. The lack of an explicit yield statement does not prevent coroutines from switching. In fact, greenlets are quite powerful here, more powerful than what's offered by Twisted and Tornado, because they don't have to yield directly to the caller. You could imitate CPS which is impossible with the explicit context switch model. I am not disagreeing with you on this point, btw, just trying to be more explicit for other readers about the cooperatively scheduled nature. Interested readers might want to check this out: http://en.wikipedia.org/wiki/Computer_multitasking#Cooperative_multitasking.2Ftime-sharing http://en.wikipedia.org/wiki/Computer_multitasking#Cooperati... > I'm curious how large gevent code bases are in the wild Well, eventlet itself was written to support Second Life and sits behind multiple big video games. Gevent is in production use for many large companies, most notably Spotify. They are moving to gevent from twisted for performance and cleaner code. > I didn't realize when I started using Erlang I signed a contract saying that. I also write Ocaml, so I guess any criticism I levy against Java is a product of that too, not rational thought. My statement is facetious, but the reality is that Erlang got concurrency right and Python is trying to glue concurrency ideas into the language as an afterthought. If Python didn't have it's lousy GIL we wouldn't have to pull such tricks to work concurrency into a single threaded system. Alas, it has the GIL. Now, to conclude some of my thoughts, I believe gevent is an excellent way to build web systems. The coroutines are short-lived, exist to fetch some data with an easy nonblocking interface and then concat some strings together to produce the web page / API output. If folks were building CPU bound systems it would straight-up be a mistake to use Python for that. Ocaml could be a better choice, alas, as you say, it has the same issue with a GIL though the computations would execute a LOT faster.