How to collect as much as possible with collectgarbage() ?

[Ignacio Burgueño <ignaciob@...>]

Hi everyone. Maybe this topic has been discussed to death, but I don't 
fully understand how it is supposed to work.

I need to read a huge file from a socket, write it to disk and then call 
a library which will perform some operations on the file. That library 
requires lots of memory, and I'm trying to release as much memory as 
possible before calling into the library (I already got some "not enough 
memory" errors and I'm trying to avoid that).

Given this simple script:

local function showMem(msg)
   print( "Memory used: " .. math.floor(collectgarbage("count")) .. " 
Kb " .. (msg or ""))
end

local filename = [[c:\file_50mb.txt]]   -- a 50 megabyte file with 
random data
local f = io.open(filename, "rb")

showMem()

local data = f:read("*a")
f:close()
f = nil

showMem("before collecting")
data = nil
for i=1,20 do
   collectgarbage()
   showMem("after collecting")
end

I get the following: (under Windows XP SP3, Lua 5.1.4)

Memory used: 230 Kb
Memory used: 157204 Kb before collecting
Memory used: 12997 Kb after collecting
Memory used: 3397 Kb after collecting
Memory used: 997 Kb after collecting
Memory used: 397 Kb after collecting
Memory used: 247 Kb after collecting
Memory used: 210 Kb after collecting
Memory used: 201 Kb after collecting
Memory used: 198 Kb after collecting
Memory used: 198 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting
Memory used: 197 Kb after collecting



Calling collectgarbage does not free all possible memory, so I suppose 
that each time it is called, more stuff becomes collectable, until there 
is nothing more. Short of measuring the memory use and stopping when the 
used Kbytes don't change between two calls to collectgarbage() (ugly 
hack!) is there any other way to collect everything?

Regards,
Ignacio Burgueño