I'm using python 3.3 on a computer with 2 cores but 4 threads. I am trying to learn to use multiprocessing to speed up code, but when using it my code slows down.
To start my learning, I have made a small program:
from multiprocessing import Process
import time
def f():
s = 0
for i in range(2*10**7):
s += i
return s
if __name__ == '__main__':
t = time.time()
p1 = Process(target = f)
p2 = Process(target = f)
p3 = Process(target = f)
p4 = Process(target = f)
p1.start()
p2.start()
p3.start()
p4.start()
p1.join()
p2.join()
p3.join()
p4.join()
print (time.time()-t)
t2 = time.time()
for a in range(4):
f()
print(time.time()-t2)
Average of 3 runs, the first part with multiprocessing takes 17.15 sec, while the second part without multiprocessing takes 6.24 sec. Using the Windows Task Manager, I see that my computer is indeed using 100% CPU for the first part and only 25% for the second part, and also that I am not running out of memory.
Why is this program so much slower with multiprocessing?
