Re: Storing table references in CDATA

[Coda Highland <chighland@...>]

On Thu, May 3, 2012 at 2:17 AM, Arran Cudbard-Bell
<a.cudbardb@freeradius.org> wrote:
> Hi All,
>
> I'm currently implementing a specialised datastore using LuaJIT as part of a
> larger project.
>
> The datastore uses lua tables to represent fields and nodes, and has a set
> of functions to merge, delete and retrieve data. As these functions traverse
> the nested table structure they add information about the current node to
> stack (another array of arrays).
>
> Seems to me that it would be more efficient to preallocate all the memory
> for this stack as cdata, and also to avoid using lua tables which are fairly
> costly to create and slow to access.*
>
> Each frame in the stack contains the node (lua table) that was updated or
> traversed, the string key used to access that node in the parent, the
> template for the node, and the return value after the operation is completed
> (frames aren't popped immediately, to allow reenterant calls).
>
> The string key and the return value are easy to represent in cdata, but I
> can't figure out if it's possible to get or store pointers to lua objects.
> GC isn't a problem because the tables are referenced in other places.
>
> I've not tried embedding LUA in another application yet, so maybe what i'm
> looking for is somewhere in the C API, if this is the case then i'll gladly
> go RTFM, just want to know if it's possible?
>
> Many Thanks,
> Arran
>
> Arran Cudbard-Bell
> a.cudbardb@freeradius.org
>
> Betelwiki, Betelwiki, Betelwiki.... http://wiki.freeradius.org/ !
>
> * Slow is sort of relative here, the system can do about 130K TPS traversing
> 5 nodes, and inserting 5 fields (yey LuaJIT), but were looking for something
> in the range of 500K. Almost certainly going to have to move to using CDATA
> instead of lua tables at some point, but thats more complex, and this is
> more of a POC currently.

The "canonical" way to hold a reference to a Lua object is luaL_ref,
which puts a reference to the object in the registry and returns an
int you can use to look it up on demand.

/s/ Adam