[python-win32] The GIL and COM

[Mark Hammond]

On 22/01/2009 9:38 AM, Brad Johnson wrote:
>>> However, while it is waiting for the
>>> GIL in the UI thread (when the worker thread has it), it is locked into a
>>> WaitForSingleObject call (some layers below PyGILState_Ensure).
>> Obviously the GIL can only be held by one thread at a time.  The
>> question then is why your worker thread isn't periodically releasing the
>> GIL so the UI thread can grab it.

> I think this is the key. Since my application embeds Python, I call
> PyGILState_Ensure in the worker thread to acquire the GIL before doing anything
> related to Python.

And you release it at the end, correct?

> I understand Python releases the GIL periodically, but could
> my call be overriding that? If so, how can I run any Python code and hope to
> allow other threads to take a stab at getting the GIL?

So one your thread has acquired the GIL, one of 3 things happens:

* It returns quickly and the GIL is released.
* It executes many many instructions: every 'n' instructions Python will 
temporarily release the GIL for you.
* It calls some other blocking function.  This other blocking function 
must release the GIL before blocking, then re-aquire it when it needs to 
re-enter Python.

Each of those 3 scenarios allows the other thread to run.  You need to 
identify which of the 3 your thread falls into - I suspect it will end 
up being the final one, and the blocking functions do *not* release the 
GIL (note that all COM calls you make on any thread *will* release the 
GIL before making the actual call - it is more likely to be some 
function *you* expose to Python in a module or similar?

Cheers,

Mark