RE: Garbage collection

["Nick Trout" <ntrout@...>]

> [mailto:owner-lua-l@tecgraf.puc-rio.br] On Behalf Of Thatcher Ulrich
> Sent: Wednesday, October 23, 2002 4:23 PM

> I've been thinking about this a little -- the regular mark-sweep
> should be able to collect cyclic stuff, right?  So reference-counting
> is sort of an incremental collector for non-cyclic structures.  The
> great thing about reference counting is that it's simple and easy to
> understand.  Has David posted his patch publically?

If the patch is public then I don't know where it is. 

Maybe you could use the two together. Python has had to resort to this.
I believe their cyclic collector is a script library so Lua would have
the advantage of a faster C based one. I've no idea what algorithm they
use but I don't think its mark and sweep.
http://www.python.org/doc/current/lib/module-gc.html


> > It slightly complicates object management from the C side 
> because you
> > have to manage the reference count to Lua objects when you want to
> > reference them. There is potential for error here if don't 
> reference and
> > unreference them correctly. You may be able to hide all of 
> this in the
> 
> I think the existing Lua API should be adequate -- a lua_ref just
> increments the object's ref count when it's created via lua_ref(), and
> decrements it when it's destroyed via lua_unref().  No need to expose
> the concept of the ref count outside of the core.

I havent had time to check and I don't know if there are differences in
Lua 5, but from what I remember you could use ref and unref as you
describe. I don't think you'd have to alter their functionality as they
are reference counting/locking Lua objects already. The internals of Lua
would, I think, need some work. So this patch will be interesting to
see.

You may get problems when you get "islands" of objects which the M&S
collector would delete but have C references to and should not be
deleted. Lua_ref() locks these objects for C. There may have to be a
separate counter for Lua internally which the C count would override.

A (total) reference count method to return the number of references to
an object would still be a very useful debugging tool for making sure
that you are releasing objects when you think you are. (eg. When you do
a obj=nil).

Nick