[Python-Dev] PEP 556: Threaded garbage collection

Nick Coghlan ncoghlan at gmail.com
Fri Sep 8 13:03:33 EDT 2017

On 8 September 2017 at 08:05, Antoine Pitrou <solipsis at pitrou.net> wrote:
> Hello,
> I've written a PEP by which you can tell the GC to run in a dedicated
> thread.  The goal is to solve reentrancy issues with finalizers:
> https://www.python.org/dev/peps/pep-0556/

+1 from me for the general concept. (Minor naming idea: "inline" may
be a clearer name for the current behaviour).

One point that seems worth noting: even with cyclic GC moved out to a
separate thread, __del__ methods and weakref callbacks triggered by a
refcount going to zero will still be called inline in the current
thread. Changing that would require a tweak to the semantics of
Py_DECREF where if the GC was in threaded mode, instead of finalizing
the object immediately, it would instead be placed on a FIFO queue
where the GC thread would pick it up and then actually delete it.


> PS: I did not come up with the idea for this PEP while other people
> were having nightmares.

I dunno, debugging finalizer re-entrancy problems seems pretty
nightmarish to me ;)

Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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