Speed up ColliderTransform propagation and extract collider hierarchy logic into ColliderHierarchyPlugin
#377
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Jondolf
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C-Performance
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Hmm, I might be able to use a similar approach for the "normal" Bevy transform propagation that we run in like four other places... I'll test this in a follow-up |
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I have the follow-up implemented, and it reworks this further. I'll merge this first though, and open the follow-up right after |
Jondolf
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Jun 21, 2024
…380) # Objective #377 refactored collider hierarchy logic and extracted it into a `ColliderHierarchyPlugin<C: ScalableCollider>`, along with significantly speeding up `ColliderTransform` propagation using a `ColliderAncestor` marker component to ignore trees that don't have colliders. However, a similar marker component approach can *also* be used to speed up the many copies of transform propagation that the engine has. By limiting the propagation runs to physics entities, we could drastically reduce the overhead for scenes with lots of non-physics entities, further addressing #356. ## Solution - Extract the ancestor marker logic from `ColliderHierarchyPlugin` into an `AncestorMarkerPlugin<C: Component>` that adds an `AncestorMarker<C>` component for all ancestors of entities with the given component `C`. The logic was also refactored to be more robust and fix some edge cases and fully support hierarchy changes, along with adding more tests. - Use this plugin in `SyncPlugin` to mark rigid body ancestors. - Define custom transform propagation systems that *only* traverse trees that have physics entities (rigid bodies or colliders). See the comments in `sync.rs` above the propagation systems to see how this works. - To support custom collision backends without having to add generics to all the propagation systems, and to avoid needing to unnecessarily duplicate systems, the `ColliderBackendPlugin` now adds a general, automatically managed `ColliderMarker` component for all colliders, which allows filtering collider entities without knowing the collider type. This lets us get rid of the generics on `ColliderHierarchyPlugin`! - I also changed some scheduling and system sets in the `PrepareSet` to account for some changes and to be more logical. ## Results Test scene: 12 substeps, 1 root entity, 100,000 child entities. All entities have just a `SpatialBundle`, and one child entity also has a `Collider`. - Before [#377](#377): ~22 FPS - After [#377](#377): ~200 FPS - After this PR: ~490 FPS Of course, this scene is not very representative of an actual game scene, but it does show just how much of an impact the transform propagation was having. A *lot* of games (probably most of them) do have many more non-physics entities than physics entities, and the overhead added by the marker components and new checks should be very minimal in comparison. --- ## Migration Guide Some `PrepareSet` system sets have changed order. Before: 1. `PreInit` 2. `PropagateTransforms` 3. `InitRigidBodies` 4. `InitMassProperties` 5. `InitColliders` 6. `InitTransforms` 7. `Finalize` After: 1. `PreInit` 2. `InitRigidBodies` 3. `InitColliders` 4. `PropagateTransforms` 5. `InitMassProperties` 6. `InitTransforms` 7. `Finalize` Additionally, the `ColliderHierarchyPlugin` added in #377 no longer needs generics. The plugin is new anyway however, so this isn't really a breaking change.
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Objective
Note: This is for the
bevy-0.14branch, because it uses new observer functionality (although it's not strictly necessary). It will be merged tomainfor the upcoming release.Fixes #356.
To handle transforms in collider hierarchies properly, the engine must currently propagate a
ColliderTransformcomponent down the hierarchy several times per frame. This is made even worse by the fact that this propagation is done at every substep, not just once or twice per frame.This wouldn't be too bad if it only affected physics entities, but the current system traverses through all entities that have a
Transform. As reported in #356, this leads to absolutely awful performance for scenes with a large number of non-physics entities. This means that the engine scales very poorly with entity count, which is unacceptable.Solution
Speed up propagation with
ColliderAncestorWhen colliders are added as children, automatically mark all ancestors with the
ColliderAncestorcomponent. Similarly, clean up the markers when a collider is removed.ColliderTransformis only propagated down trees withColliderAncestorentities. This way, we can essentially skip all unnecessary propagation.In a test scene with one root entity, 100,000 child entities, and 12 substeps, I was previously getting ~22 fps even though the entities only had a
SpatialBundleand no physics components. With the changes in this PR, I now get ~200 fps. Adding colliders to individual entities doesn't impact performance in any meaningful way, as the propagation is only done for those entities.Note that this propagation will be even less of an issue in the upcoming Avian release, because there it doesn't even need to be run in the substepping loop.
The
ColliderAncestorlogic has a pretty comprehensive unit test for adding and removing colliders in a hierarchy, but I haven't verified yet if simply changing theParentwithout moving theColliderworks as expected. Adding more tests and potentially fixing these more niche cases can be done in a follow-up.Add
ColliderHierarchyPluginThe
ColliderBackendPluginwas getting big, so I decided to extract the logic related to hierarchies andColliderTransforminto a newColliderHierarchyPlugin.Ideally, you could use the plugins fully independently and for example completely abandon hierarchies if you wanted to, but currently, they are still lightly coupled. Eliminating this coupling is a task for a follow-up.
Migration Guide
Hierarchy and transform logic for colliders has been extracted from the
ColliderBackendPlugininto a newColliderHierarchyPlugin, which by default is included in thePhysicsPluginsplugin group.If you are adding plugins manually, make sure you have both if you want that functionality.