adding/removing children and memory usage

atilim · April 2012

Hi,

I've decided to do a simple test to understand if there is a memory leak when a big hierarchy of sprites is created. I created 8000 sprites with 3 level of hierarchy, attached to the stage, removed from the stage and observed the memory usage.

Here is a sample code:

collectgarbage()
collectgarbage()
collectgarbage()
collectgarbage()
 
local mem1 = collectgarbage("count")	-- initial memory usage
 
-- create 3 levels of hierarchy where each level has 20 children (= 20*20*20 sprites)
local s0 = Sprite.new()	-- this will be the root
for i=1,20 do
	local s1 = Sprite.new()
	for j=1,20 do
		local s2 = Sprite.new()
		for k=1,20 do
			local s3 = Sprite.new()
			s2:addChild(s3)
		end				
		s1:addChild(s2)
	end	
	s0:addChild(s1)
end
 
stage:addChild(s0)	-- add the root of this hierarchy to the stage
 
collectgarbage()
collectgarbage()
collectgarbage()
collectgarbage()
 
local mem2 = collectgarbage("count")	-- memory usage after creating 8000 sprites
 
s0:removeFromParent()	-- detach from the stage and make it nil
s0 = nil
 
collectgarbage()
collectgarbage()
collectgarbage()
collectgarbage()
 
local mem3 = collectgarbage("count")	-- memory usage after detaching the hierarchy from the stage
 
print(mem1, mem2, mem3)

And this is the result:

58.28125	1539.421875	59.1982421875

mem3 is a little bigger than mem1 because of some small internal allocations and it's not a memory leak.

fawzma · April 2012

what about adding event listeners to the sprites , then adding the children and removing them?

atilim · April 2012

After some tests with adding event listeners, again no leaks.

Btw, in Lua, even if you delete all the fields of a table, the table continues to occupy some memory space.

local function gc()
	collectgarbage()
	collectgarbage()
	collectgarbage()
	collectgarbage()
end
 
gc()
local mem1 = collectgarbage("count")
 
local t = {}		-- create an empty table
for i=1,10000 do 	-- put another 10000 empty tables in it
	t[i] = {}
end
 
gc()
local mem2 = collectgarbage("count")
 
for i=1,10000 do	-- empty the table (now this table has no fields)
	t[i] = nil
end
 
gc()
local mem3 = collectgarbage("count")
 
t = nil				-- set table as nil
 
gc()
local mem4 = collectgarbage("count")
 
print(mem1, mem2, mem3, mem4)

The output is

58.0234375	626.5546875	314.0546875	58.0234375

mem3 should be much more lower but Lua keeps some memory allocated internally (even if the table is totally empty).

techdojo · April 2012

I often noticed when checking code to see if there were any memory leaks (as I moved from screen to screen) that the numbers would fluctuate slightly above the expected amount and then settle down. If you do this repeatedly in a loop you can see a pattern emerge and be pretty sure you have no leaks (I also assumed it was down to some internal Lua allocation that was eventually collected later on).

atilim · April 2012

@techdojo totally agree.

techdojo · April 2012

Ironically considering Lua is supposed to be all "just forget about memory allocation and let the garbage collector sort it out", I find I actually pay more attention to making sure that not only are things removed properly but also that all potential trailing references are also cleaned up as it's easier to have a memory / resource leak in Lua than it is in C++ ???

Good memory management should be the cornerstone of ANY app, and I'd recommend all developers to make good use of the collectgarbage() function to make sure they aren't loosing any memory unexpectedly.

katze · April 2012

Hi, i dont wanted to start a new topic for this.
so i just ask here.
i was testing out if my app is creating memleaks, and so i added
print (collectgarbage("count"))
to it.
i was wondering of constantly growing up mem usage, so i tried to find out what it is, and all that is left now from the app is this:

function onEnterFrame()
print (collectgarbage("count"))
end
stage:addEventListener(Event.ENTER_FRAME, onEnterFrame)

and even only with this there is still a growing memory usage.
or is it just the consolebuffer or something?

GregBUG · April 2012

@katze

i think is related to print function... (or as you said console buffer)

try this...

function onEnterFrame()
print (collectgarbage("count"))
collectgarbage() -- clean memory garbage
end

and no memory grow...

i don't know if it's me
but Lua garbage collector is driving me crazy....
(yes... i'm memory paranoid)

b-(

ciao,
Gianluca

atilim · April 2012

Hi @katze,

It's totally related to Lua. In print function, the number returned from collectgarbage("count") is converted to string and each _unique_ string consumes some memory until they are collected.

Change the print(collectgarbage("count")) to print(math.floor(collectgarbage("count"))) and then you can see the memory remains same.

atilim · April 2012

ahh @GregBUG beats me

GregBUG · April 2012

@atilim :P

katze · April 2012

ah i did that math.floor() to get the result more readable, and i was wondering if floor is not working correectly or something, but now i see clearly!

ty, so i now all is fine

small extra question:
should i just put collectgarbage() into the onEnterFrame event function?

techdojo · April 2012

@atilim - that's a nice little "hack" for reporting memory usage, I'll have to remember that!

AlexRu · May 2012

@atilim, thanx very much. I was looking for memory leaks in my code when move from screen to screen ~X( . It was tables. I just added table = nil in REMOVED_FROM_STAGE Event and there are no leaks more.

I think It will good if "Memory usage" subchapter will in Ultimate Guide.

May anybody explain me why call collectgarbage() for 4 times in gc()??

local function gc()
	collectgarbage()
	collectgarbage()
	collectgarbage()
	collectgarbage()
end

ar2rsawseen · May 2012

@AlexRu
http://www.giderosmobile.com/forum/discussion/comment/5427

AlexRu · May 2012

@ar2rsawseen, thanx very much

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