Lua metatable Objects cannot be purge from memory? - lua

Question

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")).

Answer 2

Main.lua:
--purge stage 0:
print("set each elements metatable to nil")
for i=1, MAX_OBJ do
setmetatable(Obj_Table[i], nil);
end
According to the documentation of Lua you can not reset a tables metatable by trying to set it to nil. It just equals the function getmetatable(). So the objects keep being linked.

Answer 3

Most dynamic languages don't really release memory back to system, but keep it ready for future allocations instead. Try to create some objects after Lua reported memory decrease - Lua should use this freed memory it kept to yourself and platform memory consumption won't increase.