Attention: Here be dragons

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Thread-safe APIs

Threads

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

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

Global scope

Most Global Scope singletons are thread-safe by default. Accessing servers from threads is supported. However, for the rendering and physics servers, thread-safe operation must be enabled in the project settings first.

This makes singletons 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 or set properties from a thread, you may use call_deferred or set_deferred:

GDScriptC#

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

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:

GDScriptC#

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)

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 Godot) being 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 Sprite2D or MeshInstance3D) is not thread-safe by default. To run the rendering driver on a separate thread, set the Rendering > Driver > Thread Model project setting to Separate.

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

警告

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.

Physics

Physics simulation is not thread-safe by default. To run the physics servers on separate threads (making them thread-safe), enable the following project settings:

• PhysicsServer2D: Physics > 2D > Run on Separate Thread.

• PhysicsServer3D: Physics > 3D > Run on Separate Thread.

GDScript arrays and 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's easiest to use one thread for loading and modifying resources, and then the main thread for adding them.