user+sys time bigger than real time (with threads)

[Marco Buttu]

Let's have the following example:

$ cat foo.py
from threading import Thread, Lock

result = 0
lock = Lock()

def task():
     global result
     for i in range(10**6):
         lock.acquire()
         result += 1
         lock.release()

if __name__ == '__main__':
     t1, t2 = Thread(target=task), Thread(target=task)
     t1.start()
     t2.start()
     t1.join()
     t2.join()
     print('result:', result)

When I execute it (Python 3.6), I get a sys+user time bigger than the 
real time:

$ time python foo.py
result: 2000000

real   0m7.088s
user   0m6.597s
sys    0m5.043s

I can expect this result when I run some processes in parallel on 
different CPUs, but this code uses threads, so the GIL prevents the two 
task() functions to be executed in parallel. What am I missing?

-- 
Marco Buttu

INAF-Osservatorio Astronomico di Cagliari
Via della Scienza n. 5, 09047 Selargius (CA)
Phone: 070 711 80 217
Email: mbuttu at oa-cagliari.inaf.it