multithreading

Multithreading

Run expensive work off the main thread without corrupting the scene tree. Prefer WorkerThreadPool for short parallel jobs; reach for Thread/Mutex/Semaphore only when you need a long-lived worker.

Related skills: godot-optimization for profiling before threading, assets-pipeline for asset import, csharp-godot for C# specifics, gdscript-advanced for async/await pitfalls.

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1. Threading model & safety rules

The main thread owns the scene tree — interacting with the active scene tree is not thread-safe. Observe these doc-sourced rules:

• Servers (RenderingServer, PhysicsServer) are thread-safe only after enabling it in Project Settings (Rendering > Driver > Thread Model = Separate, Physics > {2D,3D} > Run on Separate Thread). Servers handle thousands of thread-driven instances well.
• NavigationServer2D/3D are thread-safe and thread-friendly (true parallel queries); tune Navigation > Pathfinding > Max Threads.
• AStar2D/3D/Grid2D are NOT thread-safe — one dedicated thread per object only; sharing one object across threads corrupts data.
• GDScript Array/Dictionary: reading/writing existing elements across threads is OK; resizing (add/remove) needs a Mutex.
• No GPU work off the main thread (texture creation, image read/modify) — causes RenderingServer sync stalls.
• Build scene chunks off-tree in a thread, then add them on the main thread via add_child.call_deferred() — only with a single loader thread (multiple threads risk tweaking the same cached resource → crashes).

Golden rule: Mutate the scene tree only on the main thread. From a worker, hand results back with call_deferred / set_deferred.

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2. WorkerThreadPool (preferred)

WorkerThreadPool is a global singleton with threads allocated at startup. A regular task (add_task) runs on one worker; a group task (add_group_task) is distributed across workers, calling the Callable repeatedly for each element index — great for iterating many elements. Every task must be waited on (wait_for_task_completion / wait_for_group_task_completion) or its allocated resources leak. Distributing cheap work can hurt performance — only use it for genuinely expensive work.

GDScript

var enemies = [] # Filled with enemies elsewhere.

func process_enemy_ai(enemy_index):
    var processed_enemy = enemies[enemy_index]
    # Expensive per-enemy logic...

func _process(delta):
    var task_id = WorkerThreadPool.add_group_task(process_enemy_ai, enemies.size())
    # ... other main-thread work ...
    WorkerThreadPool.wait_for_group_task_completion(task_id)
    # Safe to read results now.

C# Equivalent

private List<Node> _enemies = new(); // Filled with enemies elsewhere.

private void ProcessEnemyAI(int enemyIndex)
{
    Node processedEnemy = _enemies[enemyIndex];
    // Expensive per-enemy logic...
}

public override void _Process(double delta)
{
    long taskId = WorkerThreadPool.AddGroupTask(Callable.From<int>(ProcessEnemyAI), _enemies.Count);
    // ... other main-thread work ...
    WorkerThreadPool.WaitForGroupTaskCompletion(taskId);
    // Safe to read results now.
}

This relies on the element count staying constant during the multithreaded part.

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3. Thread / Mutex / Semaphore

Real signatures: Thread.start(callable: Callable, priority := PRIORITY_NORMAL), wait_to_finish() (blocks; join before free), is_alive(). Mutex is reentrant (lock/unlock/try_lock). Semaphore exposes wait() / post(count := 1).

GDScript

The canonical semaphore producer/consumer + clean-shutdown idiom:

var counter := 0
var mutex: Mutex
var semaphore: Semaphore
var thread: Thread
var exit_thread := false

func _ready():
    mutex = Mutex.new()
    semaphore = Semaphore.new()
    thread = Thread.new()
    thread.start(_thread_function)

func _thread_function():
    while true:
        semaphore.wait() # Block until there is work.

        mutex.lock()
        var should_exit = exit_thread
        mutex.unlock()
        if should_exit:
            break

        mutex.lock()
        counter += 1
        mutex.unlock()

func increment_counter():
    semaphore.post() # Wake the worker.

func _exit_tree():
    mutex.lock()
    exit_thread = true
    mutex.unlock()
    semaphore.post()        # Unblock so it can see exit_thread.
    thread.wait_to_finish() # Join.

C# Equivalent

Godot.Mutex/Godot.Semaphore also exist, but System.Threading is idiomatic in C#:

using Godot;
using System.Threading;

public partial class Worker : Node
{
    private int _counter;
    private readonly object _lock = new();
    private readonly SemaphoreSlim _semaphore = new(0);
    private Thread _thread;
    private volatile bool _exitThread;

    public override void _Ready()
    {
        _thread = new Thread(ThreadFunction) { IsBackground = true };
        _thread.Start();
    }

    private void ThreadFunction()
    {
        while (true)
        {
            _semaphore.Wait();           // Block until there is work.
            if (_exitThread) break;
            lock (_lock) { _counter++; }
        }
    }

    public void IncrementCounter() => _semaphore.Release(); // Wake the worker.

    public override void _ExitTree()
    {
        _exitThread = true;
        _semaphore.Release();            // Unblock so it can see _exitThread.
        _thread.Join();                  // Join.
    }
}

Thread creation is slow (especially on Windows) — pre-create before heavy work, not just-in-time. Over-locking mutexes is also costly.

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4. Handing results back: call_deferred / set_deferred

GDScript

# Unsafe from a worker thread:
world.add_child(enemy)
# Safe:
world.add_child.call_deferred(enemy)

C# Equivalent

// Unsafe from a worker thread:
world.AddChild(enemy);
// Safe — use the MethodName StringName constant, NOT "AddChild":
world.CallDeferred(Node.MethodName.AddChild, enemy);

In C#, CallDeferred("AddChild") fails — the deferred/Call/Connect APIs use Godot's snake_case names. Prefer the Node.MethodName.* constants (avoids the pitfall and an allocation).

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5. Threaded resource loading

ResourceLoader.load_threaded_request(path) starts the load. Poll load_threaded_get_status(path, progress) each frame (progress[0] is the 0–1 ratio); on THREAD_LOAD_LOADED call load_threaded_get(path). load_threaded_get blocks like load() if the load is not finished — always poll first. Statuses: THREAD_LOAD_INVALID_RESOURCE / THREAD_LOAD_IN_PROGRESS / THREAD_LOAD_FAILED / THREAD_LOAD_LOADED.

GDScript

const SCENE_PATH := "res://enemy.tscn"
var _progress: Array = []

func _ready():
    ResourceLoader.load_threaded_request(SCENE_PATH)

func _process(_delta):
    var status := ResourceLoader.load_threaded_get_status(SCENE_PATH, _progress)
    match status:
        ResourceLoader.THREAD_LOAD_IN_PROGRESS:
            $ProgressBar.value = _progress[0] * 100.0
        ResourceLoader.THREAD_LOAD_LOADED:
            var scene: PackedScene = ResourceLoader.load_threaded_get(SCENE_PATH)
            add_child(scene.instantiate())
            set_process(false)
        ResourceLoader.THREAD_LOAD_FAILED, ResourceLoader.THREAD_LOAD_INVALID_RESOURCE:
            push_error("Threaded load failed: %s" % SCENE_PATH)
            set_process(false)

C# Equivalent

private const string ScenePath = "res://enemy.tscn";
private readonly Godot.Collections.Array _progress = new();

public override void _Ready() => ResourceLoader.LoadThreadedRequest(ScenePath);

public override void _Process(double delta)
{
    var status = ResourceLoader.LoadThreadedGetStatus(ScenePath, _progress);
    switch (status)
    {
        case ResourceLoader.ThreadLoadStatus.InProgress:
            GetNode<ProgressBar>("ProgressBar").Value = (double)_progress[0] * 100.0;
            break;
        case ResourceLoader.ThreadLoadStatus.Loaded:
            var scene = (PackedScene)ResourceLoader.LoadThreadedGet(ScenePath);
            AddChild(scene.Instantiate());
            SetProcess(false);
            break;
        case ResourceLoader.ThreadLoadStatus.Failed:
        case ResourceLoader.ThreadLoadStatus.InvalidResource:
            GD.PushError($"Threaded load failed: {ScenePath}");
            SetProcess(false);
            break;
    }
}

Godot 4.7+: 4.7 shipped several threaded-load correctness fixes — load_threaded_get() deadlocks (GH-119757, GH-120077), a race in load_threaded_request() (GH-118824), and resources returned by load_threaded_get() never being unloaded (GH-119394). No API change; if you carry workarounds for rare threaded-load hangs or leaks from earlier versions, re-test on 4.7 before keeping them. The poll-before-get rule above still applies.

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6. C# concurrency: Tasks vs Godot threads

In C#, prefer System.Threading.Tasks.Task.Run / async-await for fire-and-forget CPU work; never touch Godot objects or await ToSignal(...) from a background thread — marshal results back with CallDeferred. Use WorkerThreadPool when you want Godot's pool and engine integration; use Task when you want .NET idioms. (GDScript users: use WorkerThreadPool or Thread from the sections above.)

public override void _Process(double delta)
{
    if (Input.IsActionJustPressed("compute"))
    {
        _ = System.Threading.Tasks.Task.Run(() =>
        {
            int result = ExpensiveComputation();   // Pure CPU work, no Godot objects.
            CallDeferred(MethodName.OnComputed, result); // Marshal back to main thread.
        });
    }
}

private void OnComputed(int result) => GD.Print($"Done: {result}");

Deeper: see Pitfalls & deadlocks for data races, the ERR_BUSY nested-wait deadlock, and when threading hurts.

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Implementation Checklist

• Profiled first — confirmed the work is genuinely CPU-expensive (see godot-optimization)
• Scene-tree mutations happen only on the main thread (call_deferred / set_deferred)
• Every WorkerThreadPool task is waited on (wait_for_*_completion)
• Shared state guarded by a Mutex / lock; container resizes are locked
• Threads joined (wait_to_finish / Join) before the owning node frees
• No GPU calls, no AStar sharing, no same-resource loads across threads
• Threaded loads poll status before calling load_threaded_get