On 28/07/2010 12:43, Ronald Oussoren wrote:
On 28 Jul, 2010,at 12:56 PM, Michael Foord <fuzzy...@voidspace.org.uk>
wrote:
On 28/07/2010 11:50, Nick Coghlan wrote:
> On Tue, Jul 27, 2010 at 12:33 AM, Ronald Oussoren
> <ronaldousso...@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.
>
>
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