Re: [Python-Dev] User + sys time bigger than real time, in case of no real parallelism
On Sonntag, 5. Februar 2017 07:59:04 Marco Buttu wrote:
> 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: 200
>
> real 0m7.088s
> user 0m6.597s
> sys0m5.043s
>
> 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 :(
/usr/bin/time(!) gives some insight:
$ /usr/bin/time python3 foo.py
result: 200
1.18user 1.30system 0:01.52elapsed 163%CPU (0avgtext+0avgdata
8228maxresident)k
8inputs+0outputs (0major+1151minor)pagefaults 0swaps
Obviously, the spinning happens outside the GIL...
Cheers,
Pete
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Re: [Python-Dev] User + sys time bigger than real time, in case of no real parallelism
> 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 :( With such finely intermingled thread activity, there might be a fair bit of spinlock spinning involved. Additionally, I suspect that the kernel does not track CPU time at microsecond precision and will tend to round the used times up. Obviously, this is not a reasonable way to use threads. The example is only effective at producing lots of overhead. ___ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
[Python-Dev] User + sys time bigger than real time, in case of no real parallelism
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: 200
real 0m7.088s
user 0m6.597s
sys0m5.043s
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: mbu...@oa-cagliari.inaf.it
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