I am really sorry for the OT :-( I asked elsewhere but without any answer :-( I can not figure out why in this short example the user+sys time is bigger than real time. The example executes the task() functions twice, with each execution in a separate thread. The task() just increment 10**6 times a global int: $ 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 That is usually what I can expect in case of tasks executed in parallel on different CPUs. But my example should not be the case, due to the GIL. What am I missing? Thank you very much, and sorry again for the OT :( -- Marco Buttu INAF-Osservatorio Astronomico di Cagliari Via della Scienza n. 5, 09047 Selargius (CA) Phone: 070 711 80 217 Email: mbuttu@oa-cagliari.inaf.it