Guido van Rossum, 12.08.2011 23:38:
On Fri, Aug 12, 2011 at 12:57 PM, Rene Nejsum wrote:
I think I understand the background and need for GIL. Without it Python
programs would have been cluttered with lock/synchronized statements and
C-extensions would be harder to write.
No, sorry, the first half of this is incorrect: with or without the
GIL *Python* code would need the same amount of fine-grained locking.
(The part about C extensions is correct.) I am butting in because this
is a common misunderstanding that really needs to be squashed whenever
it is aired -- the GIL does *not* help Python code to synchronize. A
thread-switch can occur between any two bytecode opcodes. Without the
GIL, atomic operations (e.g. dict lookups that doesn't require
evaluation of __eq__ or __hash__ implemented in Python) are still
supposed to be atomic.
And in this context, it's worth mentioning that even C code can be bitten
by the GIL being temporarily released when calling back into the
interpreter. Only plain C code sequences safely keep the GIL, including
many (but not all) calls to the C-API.
Stefan
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