[Python-Dev] Re: Moving towards Python 3.0 (original) (raw)

Michael Hudson mwh at python.net
Mon Jan 31 20:40:48 CET 2005

Evan Jones <ejones at uwaterloo.ca> writes:

On Jan 31, 2005, at 0:17, Guido van Rossum wrote:

The "just kidding" applies to the whole list, right? None of these strike me as good ideas, except for improvements to function argument passing. Really? You see no advantage to moving to garbage collection, nor allowing Python to leverage multiple processor environments? I'd be curious to hear your reasons why not.

Obviously, if one could wave a wand and make it so, we would. The argument about whether the cost (in backwards compatibility, portability, uniprocessor performace, developer time, etc) outweighs the benefit.

My knowledge about garbage collection is weak, but I have read a little bit of Hans Boehm's work on garbage collection. For example, his "Memory Allocation Myths and Half Truths" presentation (http://www.hpl.hp.com/personal/HansBoehm/gc/myths.ps) is quite interesting. On page 25 he examines reference counting. The biggest disadvantage mentioned is that simple pointer assignments end up becoming "increment ref count" operations as well, which can "involve at least 4 potential memory references." The next page has a micro-benchmark that shows reference counting performing very poorly.

Given the current implementations extreme malloc-happyness I posit that it would be more-or-less impossible to make any form of non-copying garabage collector go faster for Python that refcounting. I may be wrong, but I don't think so and I have actually thought about this a little bit :)

The "non-copying" bit is important for backwards compatibility of C extensions (unless there's something I don't know).

Not to mention that Python has a garbage collector anyway, so wouldn't it make sense to get rid of the reference counting?

Here you're confused. Python's cycle collector depends utterly on reference counting.

(And what is it with this "let's ditch refcounting and use a garbage collector" thing that people always wheel out? Refcounting is a form of garbage collection by most reasonable definitions, esp. when you add Python's cycle collector).

My only argument for making Python capable of leveraging multiple processor environments is that multithreading seems to be where the big performance increases will be in the next few years. I am currently using Python for some relatively large simulations, so performance is important to me.

I'm sure you're tired of hearing it, but I think processes are your friend...

Cheers, mwh

-- It is time-consuming to produce high-quality software. However, that should not alone be a reason to give up the high standards of Python development. -- Martin von Loewis, python-dev

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