WoW:Lua object memory sizes

Lua contains a fairly small set of data structures. This page aims to document how much size each structure uses, to aid addon programmers with memory optomization.

This data is from WoW 2.0, Lua 5.1

Strings

Strings are allocated and garbage collected by the lua engine. Equal strings will not take up extra space (all strings are stored by reference). When a new string is allocated in memory, it will consume (22 + length) bytes. Length is naturally the number of chars in the string.

Tables

Tables behave in different ways depending on how much data is put in them, and the style of indexing used.

A new, empty table will consume 40 bytes.

As indexes are added, the table will allocate space for new indexes at an exponential rate. For example:

local t = {}  -- 40 bytes
t[1] = true   -- alloc 1 index
t[2] = true   -- alloc 1 index
t[3] = true   -- alloc 2 indexes
t[4] = true
t[5] = true   -- alloc 4 indexes
...

If a table is indexed by sequential integers, each index will take 16 bytes (not including any memory allocated for the value). If the becomes a hash, the index size will jump to 40 bytes each. Lua is "smart" and will allocate at the 16 byte rate as much as it can. If the int sequence is broken, the new values will allocate at the 40 byte rate. For example:

local t = {}  -- 40 bytes
t[1] = true   -- 16 bytes
t[2] = true   -- 16 bytes
t[3] = true   -- 32 bytes
t[4] = true
t[8] = true   -- 40 bytes
t[9] = true   -- 40 bytes
t[10] = true  -- 80 bytes
t["x"] = true
t["y"] = true -- 160 bytes
...

Note that sequential-int and hash allocation can be intermixed. Lua will continue to allocate at the 16-byte rate as long as it possibly can.

local t = {}  -- 40 bytes
t[1] = true   -- 16 bytes
t["1"] = true -- 40 bytes
t[2] = true   -- 16 bytes
t["2"] = true -- 40 bytes
t[3] = true   -- 32 bytes
t["3"] = true -- 80 bytes

Erasing values from a table will not deallocate space used for indexes. Thus, tables don't "shrink back" when erased. However, if a value is written into the table on a new index, the table will deallocate back to fit it's new data. This is not delayed until a GC step, but rather an immediate effect. To erase and shrink a table, one can use:

for i,v in pairs(t) do t[i] = nil end
t.reset = 1
t.reset = nil

It should be noted that erasing a table is generally more expensive than collecting the table during GC. One may wish to simply allocate a new empty table.