gc problem when using lightuserdata to reference C++ objects

["Cheng, Long" <long.x.cheng@...>]

Hi all:
We are building a large mmorpg game using lua as the main language. We
have a game engine built on C++. With a customize C++/lua binding, we
could export the whole game object system to lua, and we write all the
logic control code in lua. All the C++ objects are referenced in lua as
a lightuserdata, and their life cycle are controled by lua code (objects
are created by lua call to C++, and recycled by lua GC). Now one problem
I found is that, in most cases the C++ object is very big compare with
the lightuserdata. When there are a big number of objects, most process
memory are occupied by the C++ objects, but not counted in lua VM. For
example, in a typical situation the whole server process uses around
300M bytes but in lua "collectgarbage("count")" shows only around 30M
bytes. In such case lua GC fails to reclaim the dead objects, because
lua VM does not think there are enough "wasted memory" to reclaim. I'm
just wondering is there any way I can tell the GC algorithm how much
"external" memory the lightuserdata is referencing? Or is there other
"proper" ways to handle the objects life cycle? Thanks!

Regards
Long