OK,
I think I have all the answers I needed. I am using IDs to identify
co-routines, which gives me full control over their life time as far the
application code is concerned.
By the way, why not to use object reference counts and base garbage
collection on these. As soon as the ref count goes to 0 there is no doubt
the object can be collected and it's very easy to write GC incremental or
not. Granted, Lua will be a notch slower and perhaps lager but how much
slower I wonder? Every time a new reference to the object is created the
count goes 1 up. Every time the reference being removed the count goes 1
down. Removing items off the stack would be suffering the most I would
guess. I am just wondering if you have ever considered this as an option
for Lua's GC.
Thanks you,
Alex.
-----Original Message-----
From: RLake@oxfam.org.uk [mailto:RLake@oxfam.org.uk]
Sent: Thursday, May 08, 2003 1:07 PM
To: Multiple recipients of list
Subject: Re: GC order
> You can do that. First, you must store the userdata in the coroutine
> (e.g., in its stack, or in its global table, if it has a separated
> global table). So, the userdata cannot be collected before the coroutine.
> Then, you must store an entry (userdata -> coroutine) in a table with
> weak keys (but strong values). That entry will not prevent the userdata
> from being collected. The main point here is that a weak key is only
> cleared *after* its __gc method runs. That means that, when the __gc
> method runs, you still can safely retrieve the coroutine from this
> table. (This is not by chance; it is the reason why weak keys are
> only cleared after running the __gc methods).
Sorry, Roberto, but that won't work. I have pointed this out a few times.
The weak table has a strong reference to the coroutine. Strong references
are marked and traversed. The coroutine has a reference to the userdata. So
the userdata is marked. So the reference to the coroutine is never deleted
from the weak table. Consequently neither the coroutine nor the userdata
will ever get collected.
See <http://lua-users.org/wiki/GarbageCollectingWeakTables>
gentoo lua-5.0 # bin/lua
Lua 5.0 Copyright (C) 1994-2003 Tecgraf, PUC-Rio
> -- Create an object b
> b = {}
> -- Create a which refers to b
> a = {b}
> -- create a weaktable in which b points to a
> weak = setmetatable({}, {__mode = "k"})
> weak[b] = a
> -- Get rid of the only references to a and b other than the weak one
> a, b = nil, nil
> -- what's in the table?
> table.foreach(weak, print)
table: 0x8069e38 table: 0x806a0b8
> -- garbage collect a few times
> collectgarbage()
> collectgarbage()
> collectgarbage()
> collectgarbage()
> -- what's in the table?
> table.foreach(weak, print)
table: 0x8069e38 table: 0x806a0b8
