[Python-Dev] Removing the GIL (Me, not you!)

[Justin Tulloss]

On 9/14/07, Adam Olsen <rhamph at gmail.com> wrote:

> > Could be worth a try. A first step might be to just implement
> > the atomic refcounting, and run that single-threaded to see
> > if it has terribly bad effects on performance.
>
> I've done this experiment.  It was about 12% on my box.  Later, once I
> had everything else setup so I could run two threads simultaneously, I
> found much worse costs.  All those literals become shared objects that
> create contention.


It's hard to argue with cold hard facts when all we have is raw speculation.
What do you think of a model where there is a global "thread count" that
keeps track of how many threads reference an object? Then there are
thread-specific reference counters for each object. When a thread's refcount
goes to 0, it decrefs the object's thread count. If you did this right,
hopefully there would only be cache updates when you update the thread
count, which will only be when a thread first references an object and when
it last references an object.

I mentioned this idea earlier and it's growing on me. Since you've actually
messed around with the code, do you think this would alleviate some of the
contention issues?

Justin
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