Game Loops & Timing

Fixed vs variable timestep in Bevy - stable physics, smooth rendering, and frame-rate independence.

Recipe

fn physics_step(mut q: Query<&mut Position>, time: Res<Time<Fixed>>) {
    let dt = time.delta_secs();
    for mut pos in &mut q {
        pos.0 += dt * 10.0;
    }
}
 
// App::new().add_systems(FixedUpdate, physics_step);

When to reach for this:

• Any simulation sensitive to dt (physics, networking)
• Replay and determinism requirements
• Separating render FPS from simulation Hz
• Fighting jitter when frame times spike

Working Example

use bevy::prelude::*;
 
#[derive(Component)]
struct SimPosition(f32);
 
#[derive(Component)]
struct RenderPosition(f32);
 
fn fixed_sim(mut q: Query<&mut SimPosition>, time: Res<Time<Fixed>>) {
    for mut sim in &mut q {
        sim.0 += 100.0 * time.delta_secs();
    }
}
 
fn interpolate_render(
    sim_q: Query<&SimPosition>,
    mut render_q: Query<&mut RenderPosition>,
    time: Res<Time<Fixed>>,
) {
    let alpha = time.overstep_fraction();
    for (sim, mut render) in sim_q.iter().zip(render_q.iter_mut()) {
        // Simplified: in full engine store previous sim for true lerp
        render.0 = sim.0;
        let _ = alpha;
    }
}

What this demonstrates:

• Physics mutation only in FixedUpdate
• Render-facing component updated after sim
• overstep_fraction hook for interpolation alpha

Deep Dive

How It Works

• Variable loop measures real frame time delta_secs.
• Fixed accumulator consumes real time in fixed slices (e.g., 1/60 s).
• Spiral of death if too many fixed steps per frame - cap max steps.
• Interpolation draws between previous and current physics transforms.

Schedule Map

Gotchas

• Physics in Update with delta_secs - unstable stacks. Fix: move to FixedUpdate only.
• Uncapped fixed catch-up - freeze after hitch. Fix: max substeps per frame (Bevy default helps).
• Forgetting interpolation - physics looks stuttery at 144 FPS display. Fix: lerp render transform between fixed states.
• UI tied to fixed step - sluggish menus. Fix: keep UI in Update with variable time.
• Network tick mismatch - desync. Fix: align server sim rate with Fixed Hz.

Alternatives

FAQs

Related

• Physics & Audio - fixed physics
• Bevy Engine - schedules
• Performance for Games - frame budget
• Input & Events - input timing

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.