Re: Minimising memory allocation

["Nick Trout" <nick@...>]

| Basically I want to minimise the amount of temporary memory being allocated,
| so that I can reduce the frequency GC is called (and maybe reduce the hit of
| each GC?). Can anyone give some hints on this topic?
|
| For instance, if I have local variables declared inside function A, will the
| local variables allocate memory each time A is called?

This is my understanding of what happens (so I'd like to know if I'm wrong
because I've only poked around in the Lua source a bit :-)

It depends on what type is declared. If you declare a number value then this
just goes on the stack (all of the value types are a union, with number being a
float/double and the rest being pointers to data). Everything else creates an
object which must be collected later. Have a look at lvm.c and see the script i
wrote to merge source code and vm code:
http://groups.yahoo.com/group/lua-l/message/4931 (mail me for code if
necessary) - eg. look what happens when you create a local table.

Its good to make commonly used (especially none number) information global, or
encapsulate related information in a class/table to avoid creating and
collecting them constantly. ie. if you maintain a reference to them they are not
collected (and dont need to be recreated either).

Reuse objects: eg.

function new_matrix() return { 0,0,0,0 } end
function set_identity(m) m[0],m[1],m[2],m[3] = 1,0,0,1 end
function set_identity2(m) return {1,0,0,1} end

m = new_matrix()
set_identity(m)
-- rather than
m = set_identity2()


function <2:@b.lua> (17 instructions/68 bytes at 002F3328)
1 param, 13 stacks, 1 local, 0 strings, 0 numbers, 0 functions, 3 lines
     1 [2] GETLOCAL    0 ; m
     2 [2] PUSHINT     0
     3 [2] GETLOCAL    0 ; m
     4 [2] PUSHINT     1
     5 [2] GETLOCAL    0 ; m
     6 [2] PUSHINT     2
     7 [2] GETLOCAL    0 ; m
     8 [2] PUSHINT     3
     9 [2] PUSHINT     1
    10 [2] PUSHINT     0
    11 [2] PUSHINT     0
    12 [2] PUSHINT     1
    13 [2] SETTABLE    6 1
    14 [2] SETTABLE    7 1
    15 [2] SETTABLE    8 1
    16 [2] SETTABLE    9 9
    17 [2] END

function <3:@b.lua> (8 instructions/32 bytes at 002F34A0)
1 param, 6 stacks, 1 local, 0 strings, 0 numbers, 0 functions, 3 lines
     1 [3] CREATETABLE 4
     2 [3] PUSHINT     1
     3 [3] PUSHINT     0
     4 [3] PUSHINT     0
     5 [3] PUSHINT     1
     6 [3] SETLIST     0 4
     7 [3] RETURN      1
     8 [3] END


Lunchtime: down tools. Thats all I can think of for now.

N