Python Language Cython nogil:


Example

Cython is an alternative python interpreter. It uses the GIL, but lets you disable it. See their documentation

As an example, using the code that David Beazley first used to show the dangers of threads against the GIL, we'll rewrite it using nogil:

David Beazley's code that showed GIL threading problems

from threading import Thread
import time
def countdown(n):
    while n > 0:
        n -= 1

COUNT = 10000000

t1 = Thread(target=countdown,args=(COUNT/2,))
t2 = Thread(target=countdown,args=(COUNT/2,))
start = time.time()
t1.start();t2.start()
t1.join();t2.join()
end = time.time()
print end-start

Re-written using nogil (ONLY WORKS IN CYTHON):

from threading import Thread
import time
def countdown(n):
    while n > 0:
        n -= 1

COUNT = 10000000

with nogil:
    t1 = Thread(target=countdown,args=(COUNT/2,))
    t2 = Thread(target=countdown,args=(COUNT/2,))
    start = time.time()
    t1.start();t2.start()
    t1.join();t2.join()
    
end = time.time()
print end-start

It's that simple, as long as you're using cython. Note that the documentation says you must make sure not to change any python objects:

Code in the body of the statement must not manipulate Python objects in any way, and must not call anything that manipulates Python objects without first re-acquiring the GIL. Cython currently does not check this.