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thread conditional code
Tim Peterstim.one at home.com
Sat Apr 14 17:10:41 EDT 2001
[Robin Becker]> if only one (the main) thread exists at the beginning of my critical> section and I'm not creating any threads how could one pop into> existence?It depends on the context of your Python code, which I can't guess. It'squite possible for a foreign thread to call into Python, where "foreign" ==an OS thread not created via Python's thread module. This is probablyunusual under Linux, but common for some *kinds* of Python apps on Windows.I don't know enough about what you're doing to guess. But if you have totalcontrol over all threads that may pop up in your app, I couldn't guess whyyou were looking for low-level hooks either: if you have total control overthreads, you have (or can have) total knowledge of the thread situation too.> If python threads aren't initialised and I don't do the initialisation> who else can do it?Any thread whatsoever, known to Python or not, known to you or not, dependingon the context of your app. If your app doesn't support any form of threadedcallback, this probably isn't an issue for your app.> What I'm really asking is how to avoid initialising the threading> checks without losing the possibility of locking.What are "threading checks"?> I assumed probably wrongly that there is an overhead to running> multiple threads because of the ceval thread switch tests; after> scanning the source code I suspect you have the thread switch> overhead whether or not the thread stuff is initialized and then> it makes no sense to avoid using it.The periodic eval-loop "ticker checks" occur even if you compile Pythonwithout thread support. So there is no way to avoid *that* "overhead".Fiddling with the global interpreter lock is just one of the things doneinside the ticker-check block. But when Python starts running, ceval.c'sstatic interpreter_lock pointer is NULL, and the thread-switching code isskipped so long as it remains NULL. The only way it can become non-NULL isif PyEval_InitThreads() gets called. Python calls that itself when startinga new thread, and calling it is part of the dance foreign threads have to doif they want to call a Python C API function. That's it. Importing thethread or threading modules does *not* make it non-NULL, and neither does,e.g., allocating or even using (.release(), .acquire()) thread locks. So thethread-switch jitterbug does not occur until Python knows that at least twothreads exist.
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