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Date: Wed, 11 Jul 2007 14:35:34 -0700
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Subject: Re: [PHP-DEV] Mid-term Update for Cycle Collector (Google Summer of Code
 Project)
From: chris#@codewarehouse.NET (chris#)




On Wed, 11 Jul 2007 13:43:19 -0700, "David Wang" <planetbeing@gmail.com> wrote:
> Hello, everyone. My name is David Wang and I am one of the students
> participating in Google Summer of Code this year. As you may remember,
> my project is to implement a garbage collector for circular references
> in PHP. As the midterm for Summer of Code is coming up, my mentor,
> Derick Rethans, thought it would be a good idea if I shared my
> progress with the community.
> 
> As you know, PHP uses reference counting as a simple garbage
> collection system. One of the primary weaknesses of reference counting
> systems is that objects that refer indirectly or directly to
> themselves, i.e. reference cycles, are not collected. The accumulation
> of unreferenced memory that is only deallocated at the end of a
> request can be prohibitively expensive. The cycle collector I am
> implementing directly addresses this problem.
> 
> The cycle collector is, of necessity, a type of tracing garbage
> collector. However, it uses reference count information to accelerate
> the collection process. Essentially, it works by removing the internal
> reference counts (that is, references from an object within the cycle
> to another object within the cycles )from candidate cycles. In a cycle
> that can be collected, after all internal reference counts have been
> removed, the reference count of all objects within the cycle should be
> 0. The particular algorithm that I am using is the synchronous cycle
> collector described by David Bacon and V. T. Rajan in Concurrent Cycle
> Collection in Reference Counted Systems (see the article for further
> details). Various optimizations allow the cycle collector to be run
> relatively infrequently and execute speedily when it does run.
> 
> Note that because PHP is designed to be single-threaded, a synchronous
> algorithm is used which does pause program execution for cycle
> collection when cycle collection becomes necessary. If a program has
> no reference cycles, the cycle collector does not run at all.
> 
> The initial phase of implementation is complete, and I am currently
> profiling, optimizing and trying the modified version of PHP on
> various test programs in an effort to find bugs. Meanwhile, here are
> some recent benchmarks.
> 
> Some aspects of the eZ Components (http://ez.no/ezcomponents,
> available from SVN with the instructions here:
> http://ez.no/community/articles/an_introduction_to_ez_components/installation)
> testing suite use reference cycles heavily. The Template test uses
> circular references the most frequently, the Graph test also uses
> circular references, albeit less heavily.
> 
> On the Graph test, maximum memory usage with unmodified PHP was 133.9
> MB with an execution time of 8 seconds.
> On the Graph test, maximum memory usage with gc was 51.6 MB with an
> execution time of 9 seconds.
> On the Template test, maxmium memory usage with unmodified PHP was 1.5
> GB with an execution time of 30 seconds.
> On the Template test, maxmium memory usage with gc was 67.3 MB with an
> execution time of 1 minute.
> On the whole suite of tests (which includes the Graph and Template
> tests), execution time with unmodified PHP was 12:03. With cycle
> collection, it was 12:43.
> These tests were conducted on my dual core AMD X2 4400+ desktop with
> ./configure --with-gd --with-jpeg-dir --with-zlib. As you can see,
> there is the classic time vs. memory trade-off.
> 
> My project is currently being hosted on the xdebug CVS. You can get
> the latest version with the following commands:
> 
> cvs -d :pserver:srmread@cvs.xdebug.org:/repository login
> CVS password: srmread
> cvs -d :pserver:srmread@cvs.xdebug.org:/repository co circular
> Note that I'm implementing my project on top of a CVS version of PHP
> 5.2 that is a couple of weeks old. However, the cycle collector can be
> ported into other versions of PHP fairly easily as it does not affect
> the existing Zend engine code too much.
> 
> If anyone has any questions, I'll be more than happy to answer them!
> 
> Regards,
> 
> Yiduo (David) Wang
KUDOS!
> 
> --
> PHP Internals - PHP Runtime Development Mailing List
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