[ianb]

Copy-on-write lets you efficiently share objects across boundaries -- not copying everything right away (and everything in Python means everything), while also safely avoiding the sharing of state. If you are cool with threads then it's not an issue. If you can spare the overhead of a few processes then the GIL isn't an issue. I'd rather it not be a matter of tradeoffs, and I don't think either OS processes or threads are it.