Re: Trying to understand some Lua Internals ?

[Matthew Wild <mwild1@...>]

On 7 January 2012 13:02, Jeff Smith <spammealot1@live.co.uk> wrote:
> Hi
>

>
> The timings seem to suggest that is not the way it works? I am complelely
> guessing here now, but the other possibility is that when the subsequent
> lua_pcall() function runs, it must load the require files and compile to
> bytecode at that point, before it runs the bytecode. If that is the case,
> when are the require files loaded, always right at the beginning of pcall()
> or does it depend on where they are placed in the Lua source ?
>

require() is a function like any other function. Compiling a script
(loadfile) does not execute any functions, it just compiles them to
bytecode. Therefore, yes, you are on the right track - require()'d
modules will not be loaded until the pcall.

> Issue 2)
>
> While I was timing some functions  I also measured the time to create the
> lua state and open the standard Lua libs and my custom library bindings.
> This took about 200 mS, that is roughly what I would have guessed and no big
> deal.  I then also measured the time to execute the lua_close(L)
> line. Slightly to my horror this took 8 seconds !  In my system I need to
> stop and start the Lua task quite often, so this very sluggish close time is
> a nasty problem.
>
> I was anticipating the garbage collector destroying objects to be quite
> complex and slow, but didnt think it would be that bad. From a peruse of the
> code it looks like it traverses a linked link of objects and then frees them
> one at a time from the system memory pool. I put some debug code in to try
> and understand what is going on, it would seem that the lua_close() function
> triggers the destruction of over 4000 objects, (so is taking approx 2mS per
> object to free)
>
> If I measure the amount of object freeing for a single line print("Hello")
> Lua program I get
>
> objectFrees[LUA_TNIL]=0
> objectFrees[LUA_TBOOLEAN]=0
> objectFrees[LUA_TLIGHTUSERDATA]=0
> objectFrees[LUA_TNUMBER]=0
> objectFrees[LUA_TSTRING]=593
> objectFrees[LUA_TTABLE]=101
> objectFrees[LUA_TFUNCTION]=595
> objectFrees[LUA_TUSERDATA]=3
> objectFrees[LUA_TTHREAD]=0
> objectFrees[LUA_TPROTO]=2                // Anyone know what a TPROTO is, I
> cant guess what that one is ?

It's the bytecode for a function (shared by all Lua function objects
that are based on the same code).

> Total objectFrees=1,294     (this is about 400 for a standard Lua build with
> no custom libraries)
>
> So that is pretty big too, closing the lua state for a do nothing program
> would probably take 2.6 seconds or so.
>
> Any thoughts on these numbers, do these object counts look plausible ?

I don't have any code to test or time to produce code to test, but I
can say it's quite a bit higher than I would have expected for a plain
Lua state. I can't say it's wrong though, without anything else to go
on.

In general it sounds like you're on a platform with constrained
resources. Take a look at eLua, which is Lua heavily tweaked to run
efficiently on low-power devices.

Regards,
Matthew