Lag caused by adding mesh to MeshInstance3D (possibly due to the use of threads) in Godot 4.4 dev4,5,6,7 and beta1

[ogyrec-o]

Tested versions

Reproducible in: Godot 4.4 beta1, dev4,5,6,7, Godot 4.4 dev1
Not reproducible in: Godot 4.3 stable

System information

Godot v4.4.beta1 - Windows 10 (build 19045) - Multi-window, 2 monitors - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3070 (NVIDIA; 32.0.15.6636) - AMD Ryzen 7 5800X 8-Core Processor (16 threads)

Issue description

In Godot 4.4 dev1 dev4,5,6,7 and 4.4 beta1, a noticeable lag occurs when assigning a new not empty mesh to a MeshInstance3D. This happens specifically on the line:

# sb.gd
func changeMesh() -> void:
	# The lag issue is caused by this line in Godot 4.4 beta1 and dev4,5,6,7.
	# The issue persists from Godot 4.4 dev4,5,6,7 up to and including Godot 4.4 beta1.
	# If the same project is run in Godot 4.3 release, there are no issues.
	# Commenting out this line in Godot 4.4 removes the lag, suggesting the issue might be related to CSG/Mesh handling.
	# Replacing the line with add_child(CSGBox3D.new()) also causes lag in 4.4.
	# However, using add_child(Node3D.new()) works without any lag in 4.4.
	mi.mesh = BoxMesh.new()
	pass

The issue does not appear in Godot 4.3, where the same project works without any performance drops. When running the project in Godot 4.4 with this line active, there is significant lag, which can be resolved by commenting out the line.

Additionally, replacing mi.mesh = BoxMesh.new() with add_child(CSGBox3D.new()) triggers the same lag. However, using add_child(Node3D.new()) works without any lag, suggesting the problem is related to mesh handling or CSG node processing in Godot 4.4.

In my project, there are parallel tasks (threads) being executed through a task manager, similar to what was used in my main project. I’m unsure whether the presence of these tasks is contributing to this issue. I added the same multithreading setup here, but I haven't been able to determine if the lag is caused by the mesh handling or if the task manager's threads are influencing it.

https://youtu.be/Uw34gAoN7HE

Steps to reproduce

Create a new Godot project and copy the provided scripts from MRP into your project or use MRP.

• main.gd: Attach this script to a Node3D node.
• sb.gd: Used in main.gd for creating instances of SB.
• taskManager.gd: This is a helper script to manage tasks using WorkerThreadPool and should be Global.
• Open the project in the Godot editor and run the scene.

Observe the performance issue:

The project creates 255 instances of the SB node.
Each SB runs a heavy computation (simulated in the somethingProcess() method).
After computation completes, the changeMesh() method is called to change the mesh of the SB instance.
Check the lag that occurs when the changeMesh() method sets mi.mesh = BoxMesh.new(). This will introduce a noticeable lag in Godot 4.4 dev1 dev4,5,6,7 and 4.4 beta1. The lag does not occur in Godot 4.3 stable.

Test different changes:

• Comment out the line mi.mesh = BoxMesh.new(). This removes the lag.
• Replace the line with add_child(CSGBox3D.new()). This also causes the lag to appear in Godot 4.4 dev1 dev4,5,6,7 and 4.4 beta1.
• Replace it with add_child(Node3D.new()). The lag does not occur when using a simple Node3D instead of a meshes.

Minimal reproduction project (MRP)

mrp-meshIssue4.4.zip

[AThousandShips]

Interacting with the scene tree isn't thread safe in general, and where it is thread safety mechanisms are in place to handle it safely, these can introduce stutter

But this is a bit hard to judge, what happens when not using threading?

[ogyrec-o]

Interacting with the scene tree isn't thread safe in general, and where it is thread safety mechanisms are in place to handle it safely, these can introduce stutter

But this is a bit hard to judge, what happens when not using threading?

changeMesh() method is always called in the main thread. worker threads only used for simulation of computation in somethingProcess(). when thread finishes its task, it signals the main thread to call changeMesh()

i havent yet tested issue without using threads, as the MRP was designed to copy the behavior of my main project with this issue, which uses same threading logic.

i plan to investigate further and will test how the project behaves without threads to determine whether they the issue. if i find anything relevant, i'll let know.

if theres anything else youd like me to clarify or test

[AThousandShips]

Where does it transfer to the main thread, missed this when looking

Edit: Yes I can see, this might be just natural delays when updating and creating things

[ogyrec-o]

Where does it transfer to the main thread, missed this when looking

when task.completed.emit()

# taskManager.gd - line 66
func _process(_delta: float) -> void:
	mutex.lock()
	var completed_tasks: Array = []
	for task in tasks:
		if task.is_completed():
			task.completed.emit()
			tasks.erase(task)
	mutex.unlock()

# main.gd - line 20
task.completed.connect(sbCompleted.bind(sb))

[ogyrec-o]

this might be just natural delays when updating and creating things

even when i introduce significant delays between mesh changes, there is always noticeable lag when changing 1st mesh (even for something as simple as a boxmesh). interestingly, the lag only disappears once all worker threads have finished their tasks, even if only a one mesh change has been requested during that time

this behavior is absent in Godot 4.3, where the same logic works without any lag or stuttering