Re: Speed issue?

[Graham Wakefield <lists@...>]

On Mar 25, 2008, at 6:23 PM, Eugen-Andrei Gavriloaie wrote:

> > > 2. I need a method of keeping some kind of "state" or "session"  
> > > on each connection. This "session" is
> > > opaque from c++ point of view. Must be opaque because the user of  
> > > the server must write his own
> > > Lua scripts and keep his own state particular for each  
> > > application. For example, I have app1 and app2.
> > > App1 must keep an username and a passowrd in his "session" and  
> > > app2 must keep a counter of some kind.
> >
> > How about a coroutine per session: lua_newthread() & lua_resume()?  
> > Session storage would come for free in the form of any local  
> > variables.
> One little issue... each lua_newthread() pushes a lua_State on the  
> stack of the parent context. Right? And they need to stay there,  
> otherwise
> will be garbage collected. On the other hand, stack space is  
> precious and limited. And I will surly have hundreds of  
> connections... So, I need to
> pop the lua_State from the stack and keep it somewhere and make Lua  
> interpreter understand that is not allowed to garbage collect it.
>
> Am I right? Any idea how can I keep this lua_State in the parent  
> interpreter?
>
> Thank you very much for your help!

Well, I wouldn't worry too much a few hundred stack slots... but...

Typically, to avoid collection, new threads can be placed into the  
registry, or a designated table in the registry.

E.g. to create:

L2 = lua_newthread(L);
lua_pushvalue(L, true);
lua_settable(L, LUA_REGISTRYINDEX); // pops the thread from the  
parent state

.. now do things with L2, e.g. push a function and lua_resume() it.

When the session expires, you can let garbage collection occur again  
like this:

lua_pushthread(L2);
lua_pushnil(L2);
lua_settable(L2, LUA_REGISTRYINDEX);

Also note that garbage collection and LUA_REGISTRYINDEX is shared  
between the parent and all child states.