Mapgen memory optimisations

This page contains community-written advice, tips, tricks or recommendations about modding. Your mileage may vary.

This page is based on Paramat's forum topic and has been updated for Minetest 5.0.0.

Lua map generators can use excessive memory if they are not using these 3 optimisations. Not using these optimizations can eventually lead to OOM (out-of-memory) errors because the Lua mapgen wasted way too much memory. Applying this advice is strongly recommended, and should make OOM errors much less likely. But there is no guarantee this will fix all OOM errors, your mapgen might still use excessive memory for other reasons, or the computer just has very limited memory.

Perlin noise objects: Only create once

The noise object is created by minetest.get_perlin_map(). It has to be created inside register_on_generated() to be usable, but only needs to be created once, many mapgen mods create it for every mapchunk, this consumes memory unnecessarily.

Localise the noise object outside register_on_generated() and initialise it to nil. See the code below for how to create it once only. The creation of the perlin noise tables with get_3d_map_flat() etc. is done per mapchunk

-- Initialize noise objects to nil

local nobj_terrain = nil
local nobj_biome = nil

-- ...
-- ...

-- On generated function
minetest.register_on_generated(function(minp, maxp, seed)
   -- ...
   -- ...
   nobj_terrain = nobj_terrain or minetest.get_perlin_map(np_terrain, chulens3d)
   nobj_biome = nobj_biome or minetest.get_perlin_map(np_biome, chulens2d)
   
   nobj_terrain:get_3d_map_flat(minpos3d, nvals_terrain)
   nobj_biome:get2d_map_flat(minpos2d, nvals_biome)
   -- ...
   -- ...
end)

Perlin noise tables: Re-use a single table

The Lua table that stores the noise values for a mapchunk is big, especially for 3D noise (80³ = 512000 values). Many Lua mapgens are creating a new table for every mapchunk, while the previous tables are only cleared out slowly by garbage collection, resulting in a large and unnecessary memory use.

A buffer parameter was added in 0.4.13 to avoid this, a single table is re-used by overwriting the former values.

For each of the functions with an optional `buffer` parameter:  If `buffer` is not
nil, this table will be used to store the result instead of creating a new table.
 
#### Methods
* `get_2d_map_flat(pos, buffer)`: returns a flat `<size.x * size.y>` element array of 2D noise
with values starting at `pos={x=,y=}`
* `get_3d_map_flat(pos, buffer)`: Same as `get_2d_map_flat`, but 3D noise

Localise the noise buffer outside register_on_generated() Use the buffer parameter in get_3d_map_flat() etc.

-- Localise noise buffers
local nvals_terrain = {}
local nvals_biome = {}
-- ...
-- ...
-- On generated function
minetest.register_on_generated(function(minp, maxp, seed)
   -- ...
   -- ...
   nobj_terrain:get3d_map_flat(minpos3d, nvals_terrain)
   nobj_biome:get2d_map_flat(minpos2d, nvals_biome)
   -- ...
   -- ...
end)

Lua voxelmanip table: Re-use a single table

The Lua table that stores the node content IDs for a mapchunk plus the mapblock shell is big (112³ = 1404928 values). Many Lua mapgens are creating a new table for every mapchunk, while the previous tables are only cleared out slowly by garbage collection, resulting in a large and unnecessary memory use.

A buffer parameter was added in 0.4.13 to avoid this, a single table is re-used by overwriting the former values.

In Minetest 0.4.15, a buffer parameter was also added to get_param2_data().

* `get_data([buffer])`: Retrieves the node content data loaded into the `VoxelManip` object
    * returns raw node data in the form of an array of node content IDs
    * if the param `buffer` is present, this table will be used to store the result instead
* `get_param2_data([buffer])`: Gets the raw `param2` data read into the `VoxelManip` object
    * Returns an array (indices 1 to volume) of integers ranging from `0` to `255`
    * If the param `buffer` is present, this table will be used to store the result instead

Localise the data buffer outside register_on_generated(). Use the buffer in get_data() or get_param2_data().

-- Localise data buffer
local data = {}

-- On generated function
minetest.register_on_generated(function(minp, maxp, seed)
   -- ...
   -- ...
   local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
   local area = VoxelArea:new{MinEdge = emin, MaxEdge = emax}
   vm:get_data(data)
   -- ...
   -- ...
end)