除非waitall（），为什么greenpool不会在spawn_n之后调用func？

Question

This code prints nothing:

def foo(i):
    print i

def main():
    pool = eventlet.GreenPool(size=100)
    for i in xrange(100):
        pool.spawn_n(foo, i)
    while True:
        pass

But this code prints numbers:

def foo(i):
    print i

def main():
    pool = eventlet.GreenPool(size=100)
    for i in xrange(100):
        pool.spawn_n(foo, i)
    pool.waitall()
    while True:
        pass

The only difference is pool.waitall() . In my mind, waitall() means wait until all greenthreads in the pool are finished working, but an infinite loop waits for every greenthread, so pool.waitall() is not necessary.

So why does this happen?

Reference: http://eventlet.net/doc/modules/greenpool.html#eventlet.greenpool.GreenPool.waitall

Answer 1

The threads created in an eventlet GreenPool are green threads . This means that they all exist within one thread at the operating-system level, and the Python interpreter handles switching between them. This switching can only happen when one thread either yields (deliberately provides an opportunity for other threads to run) or is waiting for I/O.

When your code runs:

while True:
    pass

… that thread of execution is blocked – stuck on that code – and no other green threads can get scheduled.

When you instead run:

pool.waitall()

… eventlet makes sure that it yields while waiting.

You could emulate this same behaviour by modifying your while loop slightly to call the eventlet.sleep function, which yields:

while True:
    eventlet.sleep()

This could be useful if you wanted to do something else in the while True: loop while waiting for the threads in your pool to complete. Otherwise, just use pool.waitall() – that's what it's for.