`The GIL`_ (Global Interpreter Lock) is how Python allows multiple threads to operate at the same time. Python's

memory management isn't entirely thread-safe, so the GIL is requried to prevents multiple threads from running

the same Python code at once.

David Beazley has a great `guide`_ on how the GIL operates. He also covers the `new GIL`_ in Python 3.2. His

results show that maximizing performance in a Python application requires a strong understanding of the GIL,

how it affects your specific application, how many cores you have, and where your application bottlenecks are.

.. _`PyPy`: http://pypy.org

.. _`The GIL`: http://wiki.python.org/moin/GlobalInterpreterLock

.. _`Understanding GIL`: http://www.dabeaz.com/python/UnderstandingGIL.pdf

.. _`New GIL`: http://www.dabeaz.com/python/NewGIL.pdf

.. _`Thread State`: http://docs.python.org/c-api/init.html#threads