Python threads all executing on a single core

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?

Note that in many cases (and virtually all cases where your "expensive operation" is a calculation implemented in Python), multiple threads will not actually run concurrently due to Python's Global Interpreter Lock (GIL).

The GIL is an interpreter-level lock. This lock prevents execution of multiple threads at once in the Python interpreter. Each thread that wants to run must wait for the GIL to be released by the other thread, which means your multi-threaded Python application is essentially single threaded, right? Yes. Not exactly. Sort of.

CPython uses what’s called “operating system” threads under the covers, which is to say each time a request to make a new thread is made, the interpreter actually calls into the operating system’s libraries and kernel to generate a new thread. This is the same as Java, for example. So in memory you really do have multiple threads and normally the operating system controls which thread is scheduled to run. On a multiple processor machine, this means you could have many threads spread across multiple processors, all happily chugging away doing work.

However, while CPython does use operating system threads (in theory allowing multiple threads to execute within the interpreter simultaneously), the interpreter also forces the GIL to be acquired by a thread before it can access the interpreter and stack and can modify Python objects in memory all willy-nilly. The latter point is why the GIL exists: The GIL prevents simultaneous access to Python objects by multiple threads. But this does not save you (as illustrated by the Bank example) from being a lock-sensitive creature; you don’t get a free ride. The GIL is there to protect the interpreters memory, not your sanity.

See the Global Interpreter Lock section of Jesse Noller's post for more details.

To get around this problem, check out Python's multiprocessing module.

multiple processes (with judicious use of IPC) are[...] a much better approach to writing apps for multi-CPU boxes than threads.

-- Guido van Rossum (creator of Python)