Lua metatable Objects cannot be purge from memory?

Question

Hi there, I'm using a proprietary platform that reported memory usage in realtime on screen. I decided to use a Class.lua I found on http://lua-users.org/wiki/SimpleLuaClasses

However, I noticed memory issues when purging object created by this using a simple Account class. Specifically, I would start with say 146k of memory used, create 1000 objects of a class that just holds an integer instance variable and store each object into a table. The memory used is now 300k

I would then exit, iterating through the table and setting each element in the table to nil. But would never get back the 146k, usually after this I am left using 210k or something similar. If I run the load sequence again during the same session, it does not exceed 300k so it is not a memory leak.

I have tried creating 1000 integers in a table and setting these to nil, which does give me back 146k.

In addition I've tried a simpler class file (Account2.lua) that doesn't rely on a class.lua. This still incurs memory fragmentation but not as much as the one that uses Class.lua

Can anybody explain what is going on here? How can I purge these objects and get back the memory?

here is the code --------Class.lua------

-- class.lua
-- Compatible with Lua 5.1 (not 5.0).
--http://lua-users.org/wiki/SimpleLuaClasses
function class(base,ctor)
  local c = {}     -- a new class instance
  if not ctor and type(base) == 'function' then
      ctor = base
      base = nil
  elseif type(base) == 'table' then
   -- our new class is a shallow copy of the base class!
      for i,v in pairs(base) do
          c[i] = v
      end
      c._base = base
  end
  -- the class will be the metatable for all its objects,
  -- and they will look up their methods in it.
  c.__index = c

  -- expose a ctor which can be called by ()
  local mt = {}
  mt.__call = function(class_tbl,...)
    local obj = {}
    setmetatable(obj,c)
    if ctor then
       ctor(obj,...)
    else 
    -- make sure that any stuff from the base class is initialized!
       if base and base.init then
         base.init(obj,...)
       end
    end
    return obj
  end
  c.init = ctor
  c.instanceOf = function(self,klass)
      local m = getmetatable(self)
      while m do 
         if m == klass then return true end
         m = m._base
      end
      return false
    end
  setmetatable(c,mt)
  return c
end

--------Account.lua------

--Import Class template
require 'class'
local classname = "Account" 
    --Declare class Constructor
    Account = class(function(acc,balance)
    --Instance variables declared here.
         if(balance ~= nil)then
                     acc.balance = balance
                    else
                     --default value
                     acc.balance = 2097
                    end
                    acc.classname = classname
                 end)

--------Account2.lua------

local account2 = {}

account2.classname  = "unnamed"
account2.balance  = 2097

-----------Constructor 1
do
 local metatable = {
  __index = account2;
 }

 function Account2()
  return setmetatable({}, metatable);
 end
end

--------Main.lua------

require 'Account'
require 'Account2'

MAX_OBJ    = 5000;
test_value = 1000;
Obj_Table = {};
MODE_ACC0 = 0 --integers
MODE_ACC1 = 1 --Account
MODE_ACC2 = 2 --Account2
TEST_MODE = MODE_ACC0;

Lua_mem = 0;

function Load()
 for i=1, MAX_OBJ do
    if(TEST_MODE == MODE_ACC0 )then
        table.insert(Obj_Table, test_value);

    elseif(TEST_MODE == MODE_ACC1 )then
         table.insert(Obj_Table, Account(test_value)); --Account.lua

    elseif(TEST_MODE == MODE_ACC2 )then
         table.insert(Obj_Table, Account2()); --Account2.lua
        Obj_Table[i].balance = test_value;
    end
 end
end

function Purge()
    --metatable purge
    if(TEST_MODE ~= MODE_ACC0)then
      --purge stage 0: 
      print("set each elements metatable to nil")
      for i=1, MAX_OBJ do
        setmetatable(Obj_Table[i], nil);
      end
    end 

    --purge stage 1: 
    print("set table element to nil")
    for i=1, MAX_OBJ do
      Obj_Table[i] = nil;
    end 

    --purge stage 2: 
    print("start table.remove...");
    for i=1, MAX_OBJ do
    table.remove(Obj_Table, i);
    end 
    print("...end table.remove");

    --purge stage 3: 
    print("create new object_table {}");
    Obj_Table= {};

    --purge stage 4: 
    print("collectgarbage('collect')");
    collectgarbage('collect');


end

--Loop callback, called every tick
function OnUpdate()
   Lua_mem = collectgarbage('count');
   collectgarbage('collect');
end
--Loop rendering callback

function OnRender()
   DrawText(Lua_mem );
end
-------------------
--NOTE:
--code starts in idle awaiting input from user
--On first input, runs Load(), on exit runs Purge()
--Where DrawText() draws the string parameter passed, to screen.

---

--Update I've updated the code with suggestions from comments below, and will post my findings later today.

---

--Update 2 Well I've tried the above code, and it does seem collectgarbage("count") reports lua giving me back all the memory in all three scenarios. Here are my results for collectgarbage('count')

ACC0 - On start: 25.567K used - On Load: 89.334K used - On Purge: 25.567K used

ACC1 - On start: 25.567K used - On Load: 440.567k used - On Purge: 25.567K used

ACC2 - On start: 25.327K used - On Load: 245.34K used - On Purge: 25.327K used

Answer 1

You need to force the garbage collector to reclaim the memory (see collectgarbage("collect")).