Bevy Performance Agent Skill

Bevy performance

When to use

Load this when a system is slow or assets stay in memory. Pair it with bevy-debugging-profiling for the counters.

ECS shape

• Systems: the ECS page says systems run in parallel when they can. .chain() forces order and removes that parallelism for the chained set. Put .chain() only on the systems that must see each other's writes.
• Fixed step: movement and other gameplay that should not follow the render frame belong on Fixed / FixedMain (bevy::time), not on every Update.
• Measures: FrameTimeDiagnosticsPlugin records frame time, fps, and frame count. EntityCountDiagnosticsPlugin records entity count. SystemInformationDiagnosticsPlugin records process cpu_usage and mem_usage. LogDiagnosticsPlugin prints them. No target milliseconds are stated on those pages.
• Memory: Handle is reference counted. A strong handle kept in a long-lived Resource keeps the asset. The last strong drop removes it from Assets<T>. Assets::add does not deduplicate procedural assets. AssetServer::load reuses a handle while one remains.
• Schedule: diagnostics plugins are added on the App after DefaultPlugins. Do not invent a budget resource.

Scope

Chain only the systems that share a mutable query. Read frame diagnostics from DiagnosticsStore. Keep a strong Handle in a resource only while the asset must stay loaded. Changed<T> skips entities whose component did not change this tick.

Examples

[package]
name = "performance"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

Damage writes health. The death system is chained after it so it observes the same tick. Movement does not share that query, so it stays outside the chain.

#[derive(Component)]
struct Health(f32);

#[derive(Component)]
struct Velocity(Vec3);

fn apply_damage(mut query: Query<&mut Health>) {
    for mut health in &mut query {
        health.0 = (health.0 - 1.0).max(0.0);
    }
}

fn reap(mut commands: Commands, query: Query<(Entity, &Health)>) {
    for (entity, health) in &query {
        if health.0 <= 0.0 {
            commands.entity(entity).despawn();
        }
    }
}

fn integrate(time: Res<Time<Fixed>>, mut query: Query<(&Velocity, &mut Transform)>) {
    let dt = time.delta_secs();
    for (velocity, mut transform) in &mut query {
        transform.translation += velocity.0 * dt;
    }
}

pub struct SimPlugin;

impl Plugin for SimPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(FixedUpdate, (apply_damage, reap).chain())
            .add_systems(FixedUpdate, integrate);
    }
}

Frame diagnostics record fps. A strong mesh handle in a resource keeps that mesh loaded until the resource is dropped.

use bevy::diagnostic::{DiagnosticsStore, FrameTimeDiagnosticsPlugin, LogDiagnosticsPlugin};

#[derive(Resource)]
struct KeptMesh(Handle<Mesh>);

fn spawn_kept_mesh(mut commands: Commands, mut meshes: ResMut<Assets<Mesh>>) {
    let handle = meshes.add(Mesh::from(Cuboid::new(1.0, 1.0, 1.0)));
    commands.insert_resource(KeptMesh(handle));
}

fn read_fps(diagnostics: Res<DiagnosticsStore>) {
    let Some(fps) = diagnostics.get(&FrameTimeDiagnosticsPlugin::FPS) else {
        return;
    };
    let _value = fps.smoothed();
}

pub struct MeasurePlugin;

impl Plugin for MeasurePlugin {
    fn build(&self, app: &mut App) {
        app.add_plugins((
            FrameTimeDiagnosticsPlugin::default(),
            LogDiagnosticsPlugin::default(),
        ))
        .add_systems(Startup, spawn_kept_mesh)
        .add_systems(Update, read_fps);
    }
}

Anchors that moved this tick are the only transforms the follower reads. Changed<Transform> leaves the rest of the query idle.

#[derive(Component)]
struct Marker;

#[derive(Component)]
struct Anchor;

fn follow_changed(
    movers: Query<&Transform, (With<Anchor>, Changed<Transform>)>,
    mut markers: Query<&mut Transform, (With<Marker>, Without<Anchor>)>,
) {
    let Some(source) = movers.iter().next() else {
        return;
    };
    let translation = source.translation;
    for mut marker in &mut markers {
        marker.translation = translation;
    }
}

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

| Piece | Owner | Who may mutate | Schedule |

| --- | --- | --- | --- |

| Data on one entity | Component | One system via Query | Update or FixedMain |

| One value for the whole app | Resource | One system | Startup to insert, then the schedule that reads it |

| Spawn | Commands | The spawning system | Startup for the initial world; later schedules for runtime spawn |

| Order that must not race | .chain() | The chained systems only | Same schedule |

| Plugin boundary | Plugin | build registers systems and resources | Before the app runs |

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

| If you need | Do this | Do not |

| --- | --- | --- |

| Per-entity data | Component + Query | A Godot node script |

| App-wide config or clock | Resource | Copy the value onto every entity |

| Stable simulation step | FixedMain and the Fixed clock | Tie simulation to raw frame Update |

| Startup spawn | Commands in Startup | Spawn from a render system |

| Cross-system notice that is not shared state | Message or observer slot in bevy-events-messaging | A new event type invented in this file |

| An API outside the documented set | Check the official docs first | A made-up crate or API |

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.
• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.
• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No Godot servers, visibility notifiers, or profiler monitors. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No culling or batching API this file did not open. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No third-party profiler crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Official pages stay on the unversioned latest branch.

Official Documentation

• ECS - Parallel systems and .chain().
• Time examples - Fixed and FixedMain.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Who owns which piece of state.
• bevy-ecs-composition - Systems and .chain().
• bevy-debugging-profiling - How those diagnostics are plugged in.

Complements

• bevy-asset-scenes - When handles are loaded and dropped.
• bevy-character-movement - Work that belongs on the fixed timestep.
• godot-performance-optimization - Performance on the Godot engine, same depth.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.