Basically, it depends on your language implementation, and whether its implementation of threading is based on OS/hardware threads (and therefore multiprocessin

Basically, it depends on your language implementation, and whether its implementation of threading is based on OS/hardware threads (and therefore multiprocessing) or on software ("green") threads. Often if OS threads are supported, it's actually a hybrid solution. There are a number of Common Lisp implementations that support MP, often on only some target OSes, and there are a number that don't. To my knowledge, (I'm not a Python hacker!) vanilla Python doesn't support MP, whereas I suspect that JPython and IronPython support the MP infrastructure that their underlying VMs provide. Software written for green threads may often fail mysteriously on OS thread implementations. Likewise, green-threaded software may need explicit scheduling, which code written for OS threads will generally not be doing.