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herviou wrote:
> This weekend I have 
> pointed out that it can cost a non negligeable time to call the lua vm. 

Umm, this is usually not an issue? Lua can do several million
calls per second on a (slow) Pentium III.

Maybe your definition of "call" is different. Parsing a script
should only be done once (and closures created only once, too).
The resulting functions should be put into a table or an upvalue
of a C closure.

Something like:

  ... loop ... {
    lua_rawgeti(L, table_idx, idx);  // Push table outside loop/use upvalue
    ... or ...
    lua_pushvalue(L, lua_upvalueindex(idx));  // For low # of functions
    lua_call(L, nargs, nresults);

But avoid doing any of these on every iteration:

  lua_open() ... lua_close()      // Recreates a Lua universe every time.
  lua_load()/luaL_load*()         // Loads/parses a script every time.
  lua_pushcfunction()/lua_pushclosure() // Creates a C closure every time.
  lua_getglobal()/lua_setglobal() // Accesses a global every time.

There are more things to avoid on the Lua side of the code, too.
There's a Wiki page:

Even more ideas can be found by reading through fully optimized
Lua code. Shameless plug (I wrote most of these):

> I'm working with Lua for creating IA in virtual drivers in a
> traffic road simulator.

BTW: this sounds like you should investigate Lua coroutines, too.
Look through the archive about Lua coroutines for AI in games.