Attention: Here be dragons

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Thread-safe APIs¶

Threads¶

Threads are used to balance processing power across CPUs and cores. Redot supports multithreading, but not in the whole engine.

Below is a list of ways multithreading can be used in different areas of Redot.

Global scope¶

Global Scope singletons are all thread-safe. Accessing servers from threads is supported (for RenderingServer and Physics servers, ensure threaded or thread-safe operation is enabled in the project settings!).

This makes them ideal for code that creates dozens of thousands of instances in servers and controls them from threads. Of course, it requires a bit more code, as this is used directly and not within the scene tree.

Scene tree¶

Interacting with the active scene tree is NOT thread-safe. Make sure to use mutexes when sending data between threads. If you want to call functions from a thread, the call_deferred function may be used:

# Unsafe:
node.add_child(child_node)
# Safe:
node.add_child.call_deferred(child_node)

// Unsafe:
node.AddChild(childNode);
// Safe:
node.CallDeferred(Node.MethodName.AddChild, childNode);

However, creating scene chunks (nodes in tree arrangement) outside the active tree is fine. This way, parts of a scene can be built or instantiated in a thread, then added in the main thread:

var enemy_scene = load("res://enemy_scene.scn")
var enemy = enemy_scene.instantiate()
enemy.add_child(weapon) # Set a weapon.
world.add_child.call_deferred(enemy)

PackedScene enemyScene = GD.Load<PackedScene>("res://EnemyScene.scn");
Node enemy = enemyScene.Instantiate<Node>();
enemy.AddChild(weapon);
world.CallDeferred(Node.MethodName.AddChild, enemy);

Still, this is only really useful if you have one thread loading data. Attempting to load or create scene chunks from multiple threads may work, but you risk resources (which are only loaded once in Redot) tweaked by the multiple threads, resulting in unexpected behaviors or crashes.

Only use more than one thread to generate scene data if you really know what you are doing and you are sure that a single resource is not being used or set in multiple ones. Otherwise, you are safer just using the servers API (which is fully thread-safe) directly and not touching scene or resources.

Rendering¶

Instancing nodes that render anything in 2D or 3D (such as Sprite) is not thread-safe by default. To make rendering thread-safe, set the Rendering > Driver > Thread Model project setting to Multi-Threaded.

Note that the Multi-Threaded thread model has several known bugs, so it may not be usable in all scenarios.

You should avoid calling functions involving direct interaction with the GPU on other threads, such as creating new textures or modifying and retrieving image data, these operations can lead to performance stalls because they require synchronization with the RenderingServer, as data needs to be transmitted to or updated on the GPU.

GDScript arrays, dictionaries¶

In GDScript, reading and writing elements from multiple threads is OK, but anything that changes the container size (resizing, adding or removing elements) requires locking a mutex.

Resources¶

Modifying a unique resource from multiple threads is not supported. However handling references on multiple threads is supported, hence loading resources on a thread is as well - scenes, textures, meshes, etc - can be loaded and manipulated on a thread and then added to the active scene on the main thread. The limitation here is as described above, one must be careful not to load the same resource from multiple threads at once, therefore it is easiest to use one thread for loading and modifying resources, and then the main thread for adding them.