Archetypes & Chunks

Archetypes & Chunks, the underlying foundation and storage structure.

As you look at your entities, you realise that many of them are similar. They even have an identical structure.

And this is exactly where archetypes come into play. An archetype is something like a table in a database; it contains all entities with exactly the same component structure. They are part of the World and each new combination of entities is stored in a new archetype.

You don't need to worry about creating them, it all happens on its own under the bonnet.

Archetype

So what have we learnt? The structure of an entity determines its archetype!

var dwarf = world.Create(new Dwarf(), new Position(), new Velocity());
var elf = world.Create(new Elf(), new Position(), new Velocity());

All elves with the same structure are therefore in one archetype. And all dwarves with the same structure in another.

Let's look at another example.

var dwarf = world.Create(new Dwarf(), new Position(), new Velocity());
var miningDwarf = world.Create(new Dwarf(), new Position(), new Velocity(), new Pickaxe());

Even though they are both dwarves, they are still in two different archetypes as their structure is different.

And now all entities with the same structure cuddle up very close to each other in their archetype, nice... right?

Chunk

So now you've learnt what an archetype is. But what is this chunk that was mentioned earlier?

Well, while archetypes do the grouping, chunks are the place where the entities and their components are stored in memory. This also happens under the bonnet and you don't notice much of it.

Each chunk has exactly 16KB of memory which is used entirely for entities and their components. This amount of data fits perfectly into an L1 cache and is therefore loaded incredibly quickly and efficiently when the entities and components are enumerated.

Another advantage is that each archetype has N chunks, so new chunks can be quickly allocated for more entities or existing chunks can be trimmed to free up memory. The best of the best to keep your world running!

The size of the chunks adapts dynamically, the default base size is 16KB but the archetype decides based on the composition of the entities whether larger chunks might make sense. If too few entities fit into a chunk, all chunks are enlarged to the next largest multiple of the base size. The base chunk size and the minimum number of entities that fit into each chunk can be adjusted when creating the World.