Optimizing multiprocessing.Pool with expensive initialization

most obvious, lazy load

_foo = None
def f(y):
    global _foo
    if not _foo:
       _foo = Foo()
    return _foo.run(y)

The intended way to deal with things like this is via the optional initializer and initargs arguments to the Pool() constructor. They exist precisely to give you a way to do stuff exactly once when a worker process is created. So, e.g., add:

def init():
    global foo
    foo = Foo()

and change the Pool creation to:

pool = mp.Pool(4, initializer=init)

If you needed to pass arguments to your per-process initialization function, then you'd also add an appropriate initargs=... argument.

Note: of course you should also remove the

foo = Foo()

line from f(), so that your function uses the global foo created by init().