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Parallelized transform propagation (#4775)

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# Objective
Fixes #4697. Hierarchical propagation of properties, currently only Transform -> GlobalTransform, can be a very expensive operation. Transform propagation is a strict dependency for anything positioned in world-space. In large worlds, this can take quite a bit of time, so limiting it to a single thread can result in poor CPU utilization as it bottlenecks the rest of the frame's systems.

## Solution

 - Move transforms without a parent or a child (free-floating (Global)Transform) entities into a separate parallel system.
 - Chunk the hierarchy based on the root entities and process it in parallel with `Query::par_for_each_mut`. 
 - Utilize the hierarchy's specific properties introduced in #4717 to allow for safe use of `Query::get_unchecked` on multiple threads. Assuming each child is unique in the hierarchy, it is impossible to have an aliased `&mut GlobalTransform` so long as we verify that the parent for a child is the same one propagated from.

---

## Changelog
Removed: `transform_propagate_system` is no longer `pub`.
This commit is contained in:
James Liu 2022-11-21 18:18:38 +00:00
parent 585dac0582
commit eaeba0866d
5 changed files with 126 additions and 59 deletions
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3
crates/bevy_transform/Cargo.toml
View file

@ -17,5 +17,8 @@ bevy_math = { path = "../bevy_math", version = "0.9.0" }
bevy_reflect = { path = "../bevy_reflect", version = "0.9.0", features = ["bevy"] }
serde = { version = "1", features = ["derive"], optional = true }
[dev_dependencies]
bevy_tasks = { path = "../bevy_tasks", version = "0.9.0-dev" }
[features]
serialize = ["dep:serde", "bevy_math/serialize"]

4
crates/bevy_transform/src/components/global_transform.rs
View file

@ -19,8 +19,8 @@ use bevy_reflect::{std_traits::ReflectDefault, FromReflect, Reflect};
///
/// [`GlobalTransform`] is the position of an entity relative to the reference frame.
///
/// [`GlobalTransform`] is updated from [`Transform`] in the system
/// [`transform_propagate_system`](crate::transform_propagate_system).
/// [`GlobalTransform`] is updated from [`Transform`] in the systems labeled
/// [`TransformPropagate`](crate::TransformSystem::TransformPropagate).
///
/// This system runs in stage [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate). If you
/// update the [`Transform`] of an entity in this stage or after, you will notice a 1 frame lag

4
crates/bevy_transform/src/components/transform.rs
View file

@ -20,8 +20,8 @@ use std::ops::Mul;
///
/// [`GlobalTransform`] is the position of an entity relative to the reference frame.
///
/// [`GlobalTransform`] is updated from [`Transform`] in the system
/// [`transform_propagate_system`](crate::transform_propagate_system).
/// [`GlobalTransform`] is updated from [`Transform`] in the systems labeled
/// [`TransformPropagate`](crate::TransformSystem::TransformPropagate).
///
/// This system runs in stage [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate). If you
/// update the [`Transform`] of an entity in this stage or after, you will notice a 1 frame lag

18
crates/bevy_transform/src/lib.rs
View file

@ -1,10 +1,10 @@
#![warn(missing_docs)]
#![warn(clippy::undocumented_unsafe_blocks)]
#![doc = include_str!("../README.md")]
/// The basic components of the transform crate
pub mod components;
mod systems;
pub use crate::systems::transform_propagate_system;
#[doc(hidden)]
pub mod prelude {
@ -32,8 +32,8 @@ use prelude::{GlobalTransform, Transform};
///
/// [`GlobalTransform`] is the position of an entity relative to the reference frame.
///
/// [`GlobalTransform`] is updated from [`Transform`] in the system
/// [`transform_propagate_system`].
/// [`GlobalTransform`] is updated from [`Transform`] in the systems labeled
/// [`TransformPropagate`](crate::TransformSystem::TransformPropagate).
///
/// This system runs in stage [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate). If you
/// update the [`Transform`] of an entity in this stage or after, you will notice a 1 frame lag
@ -91,11 +91,19 @@ impl Plugin for TransformPlugin {
// add transform systems to startup so the first update is "correct"
.add_startup_system_to_stage(
StartupStage::PostStartup,
systems::transform_propagate_system.label(TransformSystem::TransformPropagate),
systems::sync_simple_transforms.label(TransformSystem::TransformPropagate),
)
.add_startup_system_to_stage(
StartupStage::PostStartup,
systems::propagate_transforms.label(TransformSystem::TransformPropagate),
)
.add_system_to_stage(
CoreStage::PostUpdate,
systems::transform_propagate_system.label(TransformSystem::TransformPropagate),
systems::sync_simple_transforms.label(TransformSystem::TransformPropagate),
)
.add_system_to_stage(
CoreStage::PostUpdate,
systems::propagate_transforms.label(TransformSystem::TransformPropagate),
);
}
}

156
crates/bevy_transform/src/systems.rs
View file

@ -2,74 +2,117 @@ use crate::components::{GlobalTransform, Transform};
use bevy_ecs::prelude::{Changed, Entity, Query, With, Without};
use bevy_hierarchy::{Children, Parent};
/// Update [`GlobalTransform`] component of entities that aren't in the hierarchy
pub fn sync_simple_transforms(
mut query: Query<
(&Transform, &mut GlobalTransform),
(Changed<Transform>, Without<Parent>, Without<Children>),
>,
) {
query.par_for_each_mut(1024, |(transform, mut global_transform)| {
*global_transform = GlobalTransform::from(*transform);
});
}
/// Update [`GlobalTransform`] component of entities based on entity hierarchy and
/// [`Transform`] component.
pub fn transform_propagate_system(
pub fn propagate_transforms(
mut root_query: Query<
(
Option<(&Children, Changed<Children>)>,
Entity,
&Children,
&Transform,
Changed<Transform>,
Changed<Children>,
&mut GlobalTransform,
Entity,
),
Without<Parent>,
>,
mut transform_query: Query<(
&Transform,
Changed<Transform>,
&mut GlobalTransform,
&Parent,
)>,
transform_query: Query<(&Transform, Changed<Transform>, &mut GlobalTransform), With<Parent>>,
parent_query: Query<&Parent>,
children_query: Query<(&Children, Changed<Children>), (With<Parent>, With<GlobalTransform>)>,
) {
for (children, transform, transform_changed, mut global_transform, entity) in
root_query.iter_mut()
{
let mut changed = transform_changed;
if transform_changed {
*global_transform = GlobalTransform::from(*transform);
}
root_query.par_for_each_mut(
// The differing depths and sizes of hierarchy trees causes the work for each root to be
// different. A batch size of 1 ensures that each tree gets it's own task and multiple
// large trees are not clumped together.
1,
|(entity, children, transform, mut changed, children_changed, mut global_transform)| {
if changed {
*global_transform = GlobalTransform::from(*transform);
}
if let Some((children, changed_children)) = children {
// If our `Children` has changed, we need to recalculate everything below us
changed |= changed_children;
for child in children {
let _ = propagate_recursive(
changed |= children_changed;
for child in children.iter() {
propagate_recursive(
&global_transform,
&mut transform_query,
&transform_query,
&parent_query,
&children_query,
*child,
entity,
*child,
changed,
);
}
}
}
},
);
}
fn propagate_recursive(
parent: &GlobalTransform,
transform_query: &mut Query<(
&Transform,
Changed<Transform>,
&mut GlobalTransform,
&Parent,
)>,
unsafe_transform_query: &Query<
(&Transform, Changed<Transform>, &mut GlobalTransform),
With<Parent>,
>,
parent_query: &Query<&Parent>,
children_query: &Query<(&Children, Changed<Children>), (With<Parent>, With<GlobalTransform>)>,
entity: Entity,
expected_parent: Entity,
entity: Entity,
mut changed: bool,
// We use a result here to use the `?` operator. Ideally we'd use a try block instead
) -> Result<(), ()> {
) {
let Ok(actual_parent) = parent_query.get(entity) else {
panic!("Propagated child for {:?} has no Parent component!", entity);
};
assert_eq!(
actual_parent.get(), expected_parent,
"Malformed hierarchy. This probably means that your hierarchy has been improperly maintained, or contains a cycle"
);
let global_matrix = {
let (transform, transform_changed, mut global_transform, child_parent) =
transform_query.get_mut(entity).map_err(drop)?;
// Note that for parallelising, this check cannot occur here, since there is an `&mut GlobalTransform` (in global_transform)
assert_eq!(
child_parent.get(), expected_parent,
"Malformed hierarchy. This probably means that your hierarchy has been improperly maintained, or contains a cycle"
);
let Ok((transform, transform_changed, mut global_transform)) =
// SAFETY: This call cannot create aliased mutable references.
// - The top level iteration parallelizes on the roots of the hierarchy.
// - The above assertion ensures that each child has one and only one unique parent throughout the entire
// hierarchy.
//
// For example, consider the following malformed hierarchy:
//
// A
// / \
// B C
// \ /
// D
//
// D has two parents, B and C. If the propagation passes through C, but the Parent component on D points to B,
// the above check will panic as the origin parent does match the recorded parent.
//
// Also consider the following case, where A and B are roots:
//
// A B
// \ /
// C D
// \ /
// E
//
// Even if these A and B start two separate tasks running in parallel, one of them will panic before attempting
// to mutably access E.
(unsafe { unsafe_transform_query.get_unchecked(entity) }) else {
return;
};
changed |= transform_changed;
if changed {
*global_transform = parent.mul_transform(*transform);
@ -77,20 +120,22 @@ fn propagate_recursive(
*global_transform
};
let (children, changed_children) = children_query.get(entity).map_err(drop)?;
let Ok((children, changed_children)) = children_query.get(entity) else {
return
};
// If our `Children` has changed, we need to recalculate everything below us
changed |= changed_children;
for child in children {
let _ = propagate_recursive(
propagate_recursive(
&global_matrix,
transform_query,
unsafe_transform_query,
parent_query,
children_query,
*child,
entity,
*child,
changed,
);
}
Ok(())
}
#[cfg(test)]
@ -99,9 +144,10 @@ mod test {
use bevy_ecs::prelude::*;
use bevy_ecs::system::CommandQueue;
use bevy_math::vec3;
use bevy_tasks::{ComputeTaskPool, TaskPool};
use crate::components::{GlobalTransform, Transform};
use crate::systems::transform_propagate_system;
use crate::systems::*;
use crate::TransformBundle;
use bevy_hierarchy::{BuildChildren, BuildWorldChildren, Children, Parent};
@ -110,10 +156,12 @@ mod test {
#[test]
fn did_propagate() {
ComputeTaskPool::init(TaskPool::default);
let mut world = World::default();
let mut update_stage = SystemStage::parallel();
update_stage.add_system(transform_propagate_system);
update_stage.add_system(sync_simple_transforms);
update_stage.add_system(propagate_transforms);
let mut schedule = Schedule::default();
schedule.add_stage(Update, update_stage);
@ -152,8 +200,10 @@ mod test {
#[test]
fn did_propagate_command_buffer() {
let mut world = World::default();
let mut update_stage = SystemStage::parallel();
update_stage.add_system(transform_propagate_system);
update_stage.add_system(sync_simple_transforms);
update_stage.add_system(propagate_transforms);
let mut schedule = Schedule::default();
schedule.add_stage(Update, update_stage);
@ -192,10 +242,12 @@ mod test {
#[test]
fn correct_children() {
ComputeTaskPool::init(TaskPool::default);
let mut world = World::default();
let mut update_stage = SystemStage::parallel();
update_stage.add_system(transform_propagate_system);
update_stage.add_system(sync_simple_transforms);
update_stage.add_system(propagate_transforms);
let mut schedule = Schedule::default();
schedule.add_stage(Update, update_stage);
@ -272,8 +324,10 @@ mod test {
#[test]
fn correct_transforms_when_no_children() {
let mut app = App::new();
ComputeTaskPool::init(TaskPool::default);
app.add_system(transform_propagate_system);
app.add_system(sync_simple_transforms);
app.add_system(propagate_transforms);
let translation = vec3(1.0, 0.0, 0.0);
@ -313,12 +367,14 @@ mod test {
#[test]
#[should_panic]
fn panic_when_hierarchy_cycle() {
ComputeTaskPool::init(TaskPool::default);
// We cannot directly edit Parent and Children, so we use a temp world to break
// the hierarchy's invariants.
let mut temp = World::new();
let mut app = App::new();
app.add_system(transform_propagate_system);
app.add_system(propagate_transforms)
.add_system(sync_simple_transforms);
fn setup_world(world: &mut World) -> (Entity, Entity) {
let mut grandchild = Entity::from_raw(0);