On Sun, May 26, 2013 at 10:36 PM, Peter Brooks <peter.h.m.bro...@gmail.com> wrote: > This makes complete sense - any atomic action should be atomic, so two > threads can't be doing it at the same time. They can be doing anything > else though. > > If two threads create a new object at the same time, for example, > there's potentially the problem that they'll get the same space, so > the object will be owned by both. To prevent this, the new object call > should be run in only one thread. > > If you have two objects running their methods on their own local > variables, then there's no potential for conflict, so there's no need > for them to be blocked.
That's not the way it works. The CPython interpreter always runs with the GIL held; the alternative would be to have individual mutex locks on every Python object, which is expensive for performance due to the reference counting mechanism. Python functions can't release the GIL. C functions that are called from the interpreter *can* release the GIL to allow concurrency, but are only permitted to do so as long as they're not working with Python objects, e.g. waiting on I/O or performing a long calculation on C data. There are some more detailed slides on how the GIL works at: http://www.dabeaz.com/python/UnderstandingGIL.pdf Note that the description in Part 1 describes how the GIL worked prior to Python 3.2. The new GIL is described in Part 4, but the basic underlying concept is the same. > This is an interesting subject.. There's nothing wrong with the tool > I'm using to report threads - 'Activity Monitor' is the standard > process monitor. It counts cores as 'CPUs', which seems perfectly > reasonable to me. As I said, other Unixes, such as HP-UX, do the same > thing. I have no problem with that. It's also the default in Linux, where I believe it is called "IRIX mode" (as opposed to "Solaris mode", where the difference is just a factor equal to the number of cores). What I was questioning was whether the actual number being reported was correct. If it's the standard tool for the OS, then it probably is. -- http://mail.python.org/mailman/listinfo/python-list
