[Python-Dev] GIL musings (was Re: Thoughts fresh after EuroPython)

[Michael Foord]

On 28/07/2010 12:43, Ronald Oussoren wrote:
>
>
> On 28 Jul, 2010,at 12:56 PM, Michael Foord <fuzzyman at voidspace.org.uk> 
> wrote:
>
>> On 28/07/2010 11:50, Nick Coghlan wrote:
>> > On Tue, Jul 27, 2010 at 12:33 AM, Ronald Oussoren
>> > <ronaldoussoren at mac.com> wrote:
>> >
>> >> In my opinion the GIL is a weak point of CPython and it would be 
>> nice if it
>> >> could be fixed. That is however easier said than done, a number of 
>> people
>> >> have tried in the past and ran into implementation limitations 
>> like our
>> >> refcounting garbage collector that make hard to remove the GIL without
>> >> either rewriting lots of code, or running into a brick wall
>> >> performance-wise.
>> >>
>> >> The HotPy presentation at EuroPython shows that it is possible to 
>> remove the
>> >> GIL, although at the cost of replacing the garbage collector and 
>> most likely
>> >> breaking existing C extensions (although the HotPy author seemed 
>> to have a
>> >> possible workaround for that).
>> >>
>> > This is the kind of approach that seems to hold the most promise of
>> > removing the GIL without incurring the single-threaded performance hit
>> > that has been the achilles heel of previous attempts at creating a
>> > free-threaded CPython implementation. With first IronClad and now PyPy
>> > blazing the trail in interfacing a garbage collected Python
>> > implementation with deterministic refcounting for C extension modules,
>> > it seems plausible that this kind of approach may eventually prove
>> > acceptable.
>> >
>> > Furthermore, the with statement now provides a superior alternative to
>> > application level tricks that previously relied on deterministic
>> > refcounting.
>> >
>> > While multi-threading does break down beyond a certain number of
>> > cores, it *is* possible to do safely (particularly using queues to
>> > pass references around) and can avoid plenty of serialisation overhead
>> > when dealing with sizable data structures
>> >
>>
>> Breaking binary compatibility with C extensions would be "difficult"
>> once PEP 384 (stable binary ABI) has gone into effect. As you intimate,
>> Ironclad demonstrates that C extensions *can* be interfaced with a
>> different garbage collection system whilst maintaining binary
>> compatibility. It does impose constraints however (which is why the PyPy
>> c-ext implementors chose source compatibility rather than binary
>> compatibility).
> The HotPy author mentioned that he has a scheme where refcounts could 
> be used by C extensions while the system natively uses a copying 
> collector, but I got the impression that this was not fully fleshed 
> out yet.
>
> Apple's Objective-C garbage collector has a simular feature: you can 
> use CFRetain/CFRelease to manage refcounts and the GC will only 
> collect objects where the CF reference count is 0.  This is a 
> non-copying collector in a C environment though, which makes this 
> scheme easier to implement than with a full generational copying 
> collector.
>
> It should therefore be possible to have an interpreter where the VM 
> uses a real GC and while extensions using the stable ABI could work as 
> is, but that probably requires that Py_INCREF and Py_DECREF expand 
> into function calls in the stable ABI.

Ironclad artificially inflates the refcount by one when objects are 
created. If an object is eligible for garbage collection *and* the 
refcount is 1 (so the C extension doesn't hold any references to it) 
then Ironclad decrements the refcount to zero and nature takes its 
course. That's a simplification of course (particularly around what 
happens when an object is eligible for garbage collection but the 
refcount is above 1).

This allows Py_INCREF and Py_DECREF to remain as macros - switching to 
functions would have a performance cost I guess.

Michael

>
> Implementing this would still be a significant amount of work.
>
> Ronald
>
>>
>>
>> Michael
>>
>>
>>
>> > Cheers,
>> > Nick.
>> >
>> >
>>
>>
>> -- 
>> http://www.ironpythoninaction.com/
>> http://www.voidspace.org.uk/blog
>>
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http://www.voidspace.org.uk/blog

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