Alex Rivera | Logout

Python threads all executing on a single core

Asked 2010-12-21T06:09:26.370
77

I have a Python program that spawns many threads, runs 4 at a time, and each performs an expensive operation. Pseudocode:

for object in list:
    t = Thread(target=process, args=(object))
    # if fewer than 4 threads are currently running, t.start(). Otherwise, add t to queue

But when the program is run, Activity Monitor in OS X shows that 1 of the 4 logical cores is at 100% and the others are at nearly 0. Obviously I can't force the OS to do anything but I've never had to pay attention to performance in multi-threaded code like this before so I was wondering if I'm just missing or misunderstanding something.

Thanks.

python multithreading performance
Edit
Report

2 Answers

9

Python has a Global Interpreter Lock, which can prevent threads of interpreted code from being processed concurrently.

http://en.wikipedia.org/wiki/Global_Interpreter_Lock

http://wiki.python.org/moin/GlobalInterpreterLock

For ways to get around this, try the multiprocessing module, as advised here:

Does running separate python processes avoid the GIL?

answered 2010-12-21T06:53:43.267
6

AFAIK, in CPython the Global Interpreter Lock means that there can't be more than one block of Python code being run at any one time. Although this does not really affect anything in a single processor/single-core machine, on a mulitcore machine it means you have effectively only one thread running at any one time - causing all the other core to be idle.

answered 2010-12-21T06:55:33.093

Your Answer