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Mon Aug 16 05:44:23 CEST 2004 · https://mail.python.org/pipermail/python-dev/2004-August/047855.html

On Aug 15, 2004, at 11:38 PM, Stephen J. Turnbull wrote:

>>>>>> "Raymond" == Raymond Hettinger <python at rcn.com> writes:
>
>     Raymond> Threading is a technique for decoupling tasks which are
>     Raymond> not sequentially dependent and creating the illusion of
>     Raymond> concurrency.
>
> All simultaneity is an illusion, but the concurrency of Python threads
> is not. <wink>
>
>     Raymond> Also, multiple processors cannot boost performance
>     Raymond> because Python's Global Interpreter Lock (GIL) precludes
>     Raymond> more than one thread from running in the interpreter at
>     Raymond> the same time (this was done to simplify re-entrancy
>     Raymond> issues).
>
> This confused me because "running" is ambiguous.  How about something
> like:
>
>     The Python interpreter itself is not fully reentrant, so threading
>     is accomplished by interleaving the execution of Python code from
>     different threads.  Thus, use of multiple processors cannot boost
>     performance of threaded Python code, because a single instance of
>     the interpreter can only execute code on one processor at a time.
>
> (Season to taste, fix technical inaccuracies.)  I see no reason to
> mention the GIL, an implementation detail, at all.

I think it's worth mentioning.  It's absolutely necessary to know all 
about the GIL when writing an extension that can take advantage of 
threading or embed Python correctly in a threaded application.

-bob

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[Python-Dev] Threading in the Standard Library Tour Part II