On Mon, 2010-06-28 at 19:45 -0700, sturlamolden wrote: > > > Many lockless algorithms that I have looked at thusfar require a > > > "Compare and Swap" operation. Does python have an equivalent to this? > > > Is python high level enough that it has something better than this or > > > it simply doesn't need it? > > Python does have a GIL, and contrary to the title, this has a lot to > do with the GIL. Specifically, any set of operations protected by the > GIL are atomic in the CPython interpreter. > > We can therefore get global synchronization 'for free', allowing 'lock > free' data structures that avoid using threading.Lock, and replaces it > with, well, nothing. :) > > How to do it (it takes tiny amounts of C or Cython): > > Take at look at page 14: > > http://www.dabeaz.com/python/UnderstandingGIL.pdf > > The critical part is the variable _Py_Ticker in Python's source file > ceval.c, which is declared volatile int. As it is not declared static > it is a global variable, and we can actually manipulate its value from > a C extension: > > extern volatile int _Py_Ticker; // evil!!! > > Now imagine temporarily setting _Py_Ticker to some ridiculous high > value like 0x7fffffff. That locks out all other threads, practically > forever, until we restore _Py_Ticker back to it's original value. > > What this gives us is global synchronization for free! The GIL is > there anyway, so it adds no additional overhead.
Very interesting idea. Will it work if accessed through ctypes? ticker = ctypes.c_int.in_dll(ctypes.pythonapi,"_Py_Ticker") ticker.value = 0x7fffffff Or does ctypes muck with the GIL in a way that would break this idea? Ryan -- Ryan Kelly http://www.rfk.id.au | This message is digitally signed. Please visit r...@rfk.id.au | http://www.rfk.id.au/ramblings/gpg/ for details
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