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[Sebastian Wright]

For deterministic sub-stepping, the biggest mistake I see is trying to make the render timestep the physics timestep — decouple them with a fixed accumulator loop (say 1/120s) and feed whatever remainder accumulates into discrete sub-steps. The determinism itself hinges on three things: using a fixed-point or consistently-ordered float pipeline (the same operation order on every thread/platform), avoiding any nondeterministic reduction order in your broad-phase, and never relying on sqrt/rsqrt variants that differ across SIMD widths. For the sub-steps themselves, pick your step size from velocity, not a fixed constant — dt_sub = min_distance_to_collision / |v| catches tunneling far better than arbitrary fractions, and pair every velocity integration with a split-impulse or Baumgarte positional correction (bias factor ~0.2, relaxed each iteration) so you aren’t fighting

Gaming Rig: i9-14900K & RTX 4070 Super | Work Laptop: ThinkPad X1 Carbon Gen 11 (Fedora 41)

[Sebastian Wright]

Great topic, and I’ve been benchmarking Chaos rigid body performance across a few competitive titles that use it under the hood. The key thing people miss is that the render loop and the physics simulation shouldn’t be locked to the same variable timestep — Chaos handles this with a fixed-timestep accumulator that advances the world by a constant delta (typically 1/60s or finer) regardless of where the GPU lands on frames. That’s what gives you determinism for replay sync and netcode reconciliation, which is exactly what matters in ranked play. The sub-step count is your real dial here: bumping it up improves solver stability and prevents tunneling through thin meshes at high velocities, but it’s strictly CPU-bound and linear-ish in cost. I’ve seen competitive players crash sub-step counts to the minimum that keeps objects from clipping, because every extra sub-step eats into the physics budget that’s already competing with your render thread for the same P-cores.

On

Gaming Rig: i9-14900K & RTX 4070 Super | Work Laptop: ThinkPad X1 Carbon Gen 11 (Fedora 41)

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