From: Dan Sugalski <dan@...>
Date: 2002-07-04T03:50:05+09:00
Subject: Re: Ruby implementation Q's

At 3:33 AM +0900 7/4/02, Justin Johnson wrote:
>  > I've had to deal with this particular problem while designing Parrot.
>>  The only way to do this is with reference counting, but unfortunately
>>  reference counting has a number of unfortunate downsides. (Including,
>>  but not limited to, speed penalties, lots of code complexity, a
>>  tendency to be error-prone during development, and leakage of
>>  circular structures) On the upside, reference counting does get you
>>  deterministic destruction, but I don't usually consider that
>>  worthwhile enough to offset the downsides.
>
>It's not guaranteed deterministic destruction because of the cyclic
>reference issue.

Yep. Circular references are a big downside to refcounting, though as 
they're reasonably rare (though spawning horrid hacks like weak 
references) the expense is really the bigger issue.

>Cyclic reference garbage is solved using a mark-and-sweep, is that how
>you've solved it in Parrot?  If so, reference counting means that you'll
>probably get destruction when the object becomes unreachable from the root
>set, but that may not always be the case.

Yup, we do mark and sweep for objects. (We've actually split the 
search for dead objects apart from the compaction/collection of 
unused memory, but that's as much an implementation detail as 
anything else)

>  > It's possible to offset the lack of deterministic destruction if you
>>  require a sweep for dead objects at some regular time. (End of block
>>  time is a good one, as there's generally other cleanup that needs
>>  doing then) It's not a perfect offset, but doing that is awfully
>>  expensive.
>
>It basically boils down to: "Can the object be reached from the root set?"
>Your right, thats too much to be doing too often.

It's actually remarkably cheap, over the lifetime of a program, about 
half the expense of refcounting, IIRC. (Consider how many objects 
does your program *really* have active at any one time? Usually the 
answer's "Not many")

>I like the sound of your closing paragraph and will adopt it's wisdom upon
>failing to find a good solution. :-)

Heh. When in doubt, punt and hope for the best.
-- 
                                         Dan

--------------------------------------"it's like this"-------------------
Dan Sugalski                          even samurai
dan@sidhe.org                         have teddy bears and even
                                       teddy bears get drunk