[Python-Dev] Threading idea -- exposing a global thread lock (original) (raw)

Michael Chermside mcherm at mcherm.com
Tue Mar 14 19:07:40 CET 2006

Josiah Carlson writes:

It would be nice if Jython or IronPython could (and would) implement these 'critical sections'. Whether they can or not, I think that it would be a useful feature in the CPython runtime.

The issue is not whether Jython and IronPython "will", it's whether they "can". Essentially, Jython and IronPython both use locking within their fundamental data structures. This then allows them to freely allow threads to run on multiple processors. Meanwhile, CPython lacks locks in its fundamental data structures, so it uses the GIL to ensure that code which might touch Python data structures executes on only one CPU at a time.

The concept of a "critical section" makes great sense when there is effectively only one CPU: just stop switching threads. But if code is using multiple CPUs, what does it mean? Shut down the other CPUs? To do such a thing cooperatively would require checking some master lock at every step... (a price which CPython pays, but which the implementations built on good thread-supporting VMs don't have to). To do such a thing non-cooperatively is not supported in either VM.

Since I doubt we're going to convince Sun or Microsoft to change their approach to threading, I think it is unwise to build such a feature into the Python language. Supporting it in CPython only requires (I think) no more than a very simple C extension. I think it should stay as an extension and not become part of the language.

-- Michael Chermside

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