Jython, GILs and object locking.

[Alan Kennedy]

Michael Chermside wrote:
> The GIL has the unfortunate disadvantage that it prevents multiple
> processor machines (or other modern architechures) from processing
> multiple bytecodes simultaneously in different threads. Jython does
> not have this limitation.

Michael,

That's the first definitive statement I've ever read about Jython not
being limited by a GIL on multi-processor machines.

Would the following statements be correct?

1. CPython has a Global Interpreter Lock (GIL), which allows only one
thread to execute python code at a time. This "macro-level" of locking
was chosen because empirical evidence from experimental
implementations (a long time ago, in a galaxy far, far, away) showed
that it gave better performance than approaches using more
fine-grained locking, i.e. object-level locking.

2. Jython is implemented on top of java, which has built-in support
for fine-grained locking of individual objects. Therefore, the jython
implementers chose not to use a GIL, but instead to rely on the
fine-grained locking facilities of the Java platform to protect object
integrity. So jython programs support simultaneous execution of
multiple jython threads on multiple processors (obviously assuming the
Java VM on which it is running is capable of migrating threads to
other processors).

I know I could find out the answer to this question by going directly
to the jython source, but I unfortunately don't have the time. Also, I
don't actually *need* to know the answer, since I haven't yet needed
the horsepower of a mutli-processor box. But the question is
theoretically interesting, and probably of practical interest to many
readers of c.l.py.

And if the above statements are true, then I'm sure we're going to see
an interesting discussion of why fine-grained locking is fast enough
in java, but not fast enough in C >;-)

regards,

-- 
alan kennedy
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