godot-docs/tutorials/performance/pipeline_compilations.rst

.. _doc_pipeline_compilations:

Reducing stutter from shader (pipeline) compilations
====================================================

Pipeline compilation, also commonly known as shader compilation, is an expensive
operation required by the engine to be able to draw any kind of content with the
GPU.

In more precise terms, *shader compilation* involves the translation of the GLSL
code that Godot generates into an intermediate format that can be shared across
systems (such as SPIR-V when using Vulkan). However, this format can't be used
by the GPU directly.

*Pipeline compilation* is the step where the GPU driver converts
the intermediate shader format (the result from shader compilation) to something
the GPU can actually use for rendering. Drivers usually keep a cache of
pipelines stored somewhere in the system to avoid repeating the process every
time a game is run. This cache is usually deleted when the driver is updated.

Pipelines contain more information than just the shader code, which means that
for each shader, there can be dozens of pipelines or more! This makes it
difficult for an engine to compile them ahead of time, both because it would be
very slow, and because it would take up a lot of memory. On top of that, this
step can only be performed on the user's system and it is very tough to share
the result between users unless they have the exact same hardware and driver
version.

Before Godot 4.4, there was no solution to pipeline compilation other than
generating them when an object shows up inside the camera's view, leading to the
infamous *shader stutter* or hitches that only occur during the first
playthrough. **With Godot 4.4, new mechanisms have been introduced to mitigate
stutters from pipeline compilation.**

- **Ubershaders**: Godot makes use of specialization constants, a feature that
  allows the driver to optimize a pipeline's code around a set of parameters
  such as lighting, shadow quality, etc. Specialization constants are used to
  optimize a shader by limiting unnecessary features. Changing a specialization
  constant requires recompiling the pipeline. Ubershaders are a special version
  of the shader that are able to change these constants while rendering, which
  means Godot can precompile just one pipeline ahead of time and compile the
  more optimized versions on the background during gameplay. This reduces the
  amount of pipelines that need to be created significantly.
- **Pipeline precompilation**: By using ubershaders, the engine can precompile
  pipelines ahead of time in multiple places such as when meshes are loaded or
  when nodes are added to the scene. By being part of the resource loading
  process, pipelines can even be precompiled in multiple background threads if
  possible during loading screens or even gameplay.

Starting in Godot 4.4, Godot will detect which pipelines are needed and
precompile them at load-time. This detection system is mostly automatic, but it
relies on the RenderingServer seeing evidence of all shaders, meshes, or
rendering features at load-time. For example, if you load a mesh and shader
while the game is running, the pipeline for that mesh/shader combination won't
be compiled until the mesh/shader is loaded. Similarly, things like enabling
MSAA, or instancing a VoxelGI node while the game is running will trigger
pipeline recompilations.

Pipeline precompilation monitors
--------------------------------

Compiling pipelines ahead of time is the main mechanism Godot uses to mitigate
shader stutters, but it's not a perfect solution. Being aware of the situations
that can lead to pipeline stutters can be very helpful, and the workarounds are
pretty straightforward compared to previous versions. These workarounds may be
less necessary over time with future versions of Godot as more detection
techniques are implemented.

The Godot debugger offers monitors for tracking the amount of pipelines created
by the game and the step that triggered their compilation. You can keep an eye
on these monitors as the game runs to identify potential sources of shader
stutters without having to wipe your driver cache every time you wish to test.
Sudden increases of these values outside of loading screens can show up as
hitches during gameplay the first time someone plays the game on their system.
**It is recommended you take a look at these monitors to identify possible
sources of stutter for your players**, as you might be unable to experience them
yourself without deleting your driver cache or testing on a weaker system.

.. figure:: img/pipeline_compilations_monitors.webp
   :align: center
   :alt: Screenshot of the Godot pipeline compilations monitor

   Pipeline compilations of one of the demo projects.

.. note:: We can see the pipelines compiled during gameplay and
          verify which steps could possibly cause stuttters. Note
          that these values will only increase and never go down,
          as deleted pipelines are not tracked by these monitors
          and pipelines may be erased and recreated during gameplay.

- **Canvas**: Compiled when drawing a 2D node. The engine does not currently
  feature precompilation for 2D elements and stutters will show up when the
  2D node is drawn for the first time.
- **Mesh**: Compiled as part of loading a 3D mesh and identifying what pipelines
  can be precompiled from its properties. These can lead to stutters if a mesh
  is loaded during gameplay, but they can be mitigated if the mesh is loaded by
  using a background thread. **Modifiers that are part of nodes such as material
  overrides can't be compiled on this step**.
- **Surface**: Compiled when a frame is about to be drawn and 3D objects were
  instanced on the scene tree for the first time. This can also include
  compilation for nodes that aren't even visible on the scene tree. The stutter
  will occur only on the first frame the node is added to the scene, which won't
  result in an obvious stutter if it happens right after a loading screen.
- **Draw**: Compiled on demand when a 3D object needs to be drawn and an
  ubershader was not precompiled ahead of time. The engine is unable to
  precompile this pipeline due to triggering a case that hasn't been covered
  yet or a modification that was done to the engine's code. Leads to stutters
  during gameplay. This is identical to Godot versions before 4.4. If you
  see compilations here, please
  `let the developers know <https://github.com/godotengine/godot/issues>`
  as this should never happen with the Ubershader system.
  Make sure to attach a minimal reproduction project when doing so.
- **Specialization**: Compiled in the background during gameplay to optimize the
  framerate. Unable to cause stutters, but may result in reduced framerates if
  there are many happening per frame.

Pipeline precompilation features
--------------------------------

Godot offers a lot of rendering features that are not necessarily used by every
game. Unfortunately, pipeline precompilation can't know ahead of time if a
particular feature is used by a project. Some of these features can only be
detected when a user adds a node to the scene or toggles a particular setting in
the project or the environment. The pipeline precompilation system will keep
track of these features as they're encountered for the first time and enable
precompilation of them for any meshes or surfaces that are created afterwards.

If your game makes use of these features, **make sure to have an scene that uses
them as early as possible** before loading the majority of the assets. This
scene can be very simple and will do the job as long as it uses the features the
game plans to use. It can even be rendered off-screen for at least one frame if
necessary, e.g. by covering it with a :ref:`class_ColorRect` node or
using a :ref:`class_SubViewport` located outside the window bounds.

You should also keep in mind that changing any of these features during gameplay
will result in immediate stutters. Make sure to only change these features from
configuration screens if necessary and insert loading screens and messages when
the changes are applied.

- **MSAA Level**: Enabled when the level of 3D MSAA is changed on the project
  settings. Unfortunately, different MSAA levels being used on different
  viewports will lead to stutters as the engine only keeps track of one level at
  a time to perform precompilation.
- **Reflection Probes**: Enabled when a ReflectionProbe node is placed on the
  scene.
- **Separate Specular**: Enabled when using effects like sub-surface scattering
  or a compositor effect that relies on sampling the specularity directly off
  the screen.
- **Motion Vectors**: Enabled when using effects such as TAA, FSR2 or a
  compositor effect that requires motion vectors (such as motion blur).
- **Normal and Roughness**: Enabled when using SDFGI, VoxelGI, screen-space
  reflections, SSAO, SSIL, or using the ``normal_roughness_buffer`` in a custom
  shader or :ref:`class_CompositorEffect`.
- **Lightmaps**: Enabled when a LightmapGI node is placed on the scene and a
  node uses a baked lightmap.
- **VoxelGI**: Enabled when a VoxelGI node is placed on the scene.
- **SDFGI**: Enabled when the WorldEnvironment enables SDFGI.
- **Multiview**: Enabled for XR projects.
- **16/32-bit Shadows**: Enabled when the configuration of the depth precision
  of shadowmaps is changed on the project settings.
- **Omni Shadow Dual Paraboloid**: Enabled when an omni light casts shadows and
  uses the dual paraboloid mode.
- **Omni Shadow Cubemap**: Enabled when an omni light casts shadows and uses the
  cubemap mode (which is the default).

If you witness stutters during gameplay and the monitors report a sudden
increase in compilations during the **Surface** step, it is very likely a
feature was not enabled ahead of time. Ensuring that this effect is enabled
while loading your game will likely mitigate the issue.

Pipeline precompilation instancing
----------------------------------

One common source of stutters in games is the fact that some effects are only
instanced on the scene because of interactions that only happen during gameplay.
For example, if you have a particle effect that is only added to the scene
through a script when a player does an action. Even if the scene is preloaded,
the engine might be unable to precompile the pipelines until the effect is added
to the scene at least once.

Luckily, it's possible for Godot 4.4 and later to
precompile these pipelines as long as the scene is instantiated at least once on
the scene, even if it's completely invisible or outside of the camera's view.

.. figure:: img/pipeline_compilations_hidden_node.webp
   :align: center
   :alt: Screenshot of an example of a hidden Node for an effect

   Hidden bullet node attached to the player in one of the demo projects. This
   helps the engine precompile the effect's pipelines ahead of time.

If you're aware of any effects that are added to the scene dynamically during
gameplay and are seeing sudden increases on the compilations monitor when these
effects show up, a workaround is to attach a hidden version of the effect
somewhere that is guaranteed to show up.

For example, if the player character is able to cause some sort of explosion,
you can attach the effect as a child of the player as an invisible node. Make
sure to disable the script attached to the hidden node or to hide any other
nodes that could cause issues, which can be done by enabling **Editable
Children** on the node.