Fix Physics3D running out of memory when stacking scenes with "Pause and start a new scene" by Bouh · Pull Request #9250 · 4ian/GDevelop · GitHub
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Fix Physics3D running out of memory when stacking scenes with "Pause and start a new scene" - #9250

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fix-physics3d-push-scene-oom
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Fix Physics3D running out of memory when stacking scenes with "Pause and start a new scene"#9250
Bouh wants to merge 1 commit into
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fix-physics3d-push-scene-oom

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@Bouh

@Bouh Bouh commented Oct 15, 2026 •
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Fixes #8640

Problem

The Jolt WASM module used by Physics3D has a fixed 128 MB heap (no memory growth: emscripten_resize_heap aborts with OOM). Each JoltInterface created with the default settings takes about 20 MB, so only 6 physics worlds fit.

A scene paused with "Pause and start a new scene" kept its Jolt world alive, as it was only freed when the scene was unloaded. Stacking scenes with Physics3D objects crashed the game with RuntimeError: Aborted(OOM) in new JoltInterface at the 6th stacked scene.

Fix

In Physics3DSharedData:

  • When a scene is paused, releaseJoltWorld() stores the linear and angular velocities of every body, destroys the bodies and frees the Jolt world.
  • When the scene is resumed, restoreJoltWorld() recreates the Jolt world and the bodies with their velocities.
  • PhysicsCharacter3D and PhysicsCar3D rebuild their own Jolt objects (character, vehicle constraint) lazily, with the mechanism already used when a behavior is deactivated then activated again.

What is lost across a pause: contacts in progress (collision "started" conditions trigger again after the resume), the sleeping state of bodies, and the internal state of characters (like being on the floor).

Performance before/after

Measured on the reproduction project of the issue (scene "Bug": 200 dynamic boxes, the scene pushes a new copy of itself every 0.5 s).

Measure master This PR
Stacked scenes before crash 5 (Aborted(OOM) at the 6th) 59+, no crash (the test stopped at 60)
Average frame time, first 150 frames (gameplay test) 5.98 ms 5.98 ms
Worst frame, first 150 frames (gameplay test) 71.1 ms 72 ms
Pausing a scene with 200 bodies 0.02 ms 0.45 ms
Resuming a scene with 200 bodies 0.02 ms 2.45 ms
Real preview: average FPS over 10 s crashes after about 3 s 143 FPS (6.96 ms per frame)
Real preview: worst frame (when a new scene is pushed) crashes after about 3 s about 207 ms

The cost of the fix is only paid when a scene is paused or resumed: a few milliseconds for 200 bodies. Frame time is unchanged.

How to test

  1. Open the reproduction project attached
    Repro8640-Physics3DPushSceneOOM.zip

Videos

Crash on Master
https://github.com/user-attachments/assets/fbeaaaf6-a5a8-43f6-a184-2cfa1cf9d237

This PR
https://github.com/user-attachments/assets/b0251318-29cf-4f9b-930b-510ab05333bf

@Bouh
Fix Physics3D running out of memory when stacking scenes with "Pause …
d37b3a2 
…and start a new scene"

The Jolt world of a paused scene is now freed and rebuilt when the scene is resumed, with the velocities of the bodies.
@Bouh
Bouh requested a review from 4ian as a code owner October 15, 2026 14:19
@github-actions

github-actions Bot commented Oct 15, 2026

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⏱️ Game engine benchmarks: ✅ no significant change on the 31 benchmarks

All the 31 benchmarks and how they are measured
Benchmark Base This version Change Range between rounds
≈ platformer objects jumping in loop(1000x) 129.0ms 132.8ms +2.9% -0.8% to +13.2%
≈ hitBoxesCollisionTest of 10 objects with 10 objects(10000x) 83.2ms 82.7ms -0.6% -9.4% to +3.0%
≈ hitBoxesCollisionTest of 30 objects with 30 objects(1112x) 25.3ms 24.9ms -1.4% -11.9% to +3.8%
≈ hitBoxesCollisionTest of 100 objects with 100 objects(100x) 8.261ms 8.262ms +0.0% -2.1% to +1.9%
≈ hitBoxesCollisionTest of 500 objects with 500 objects(4x) 1.867ms 1.894ms +1.4% -1.3% to +3.0%
≈ hitBoxesCollisionTest of 1000 objects with 1000 objects(1x) 1.021ms 1.035ms +1.3% -1.9% to +7.7%
≈ hitBoxesCollisionTest of 1 object with 1000 objects(1000x) 54.0ms 53.2ms -1.5% -3.8% to +0.9%
≈ hitBoxesCollisionTest of 10 objects with 1000 objects(100x) 11.2ms 11.4ms +2.2% -5.9% to +9.0%
≈ hitBoxesCollisionTest of 1000 objects with 1 object(1000x) 56.8ms 58.8ms +3.5% -7.1% to +6.6%
≈ hitBoxesCollisionTest of 500 objects with themselves(4x) 1.294ms 1.298ms +0.3% -2.3% to +3.1%
≈ setAngle and setLength(100000x) 11.4ms 11.4ms +0.0% -0.2% to +0.5%
≈ convertCoords(100000x) 21.4ms 21.6ms +0.9% -0.1% to +1.8%
≈ convertInverseCoords(100000x) 21.4ms 21.4ms -0.1% -0.5% to +0.3%
≈ collisionTest between two overlapping rectangles(120000x) 47.4ms 48.0ms +1.3% -2.7% to +3.9%
≈ collisionTest between two non overlapping rectangles(120000x) 11.1ms 11.1ms +0.2% -3.1% to +1.8%
≈ getAABB of a non rotated, default center(180000x) 19.1ms 19.0ms -0.4% -2.0% to +0.4%
≈ getAABB of a rotated, default center(180000x) 19.0ms 19.0ms -0.2% -2.0% to +0.4%
≈ getAABB of a non rotated, non default center(180000x) 20.2ms 20.1ms -0.6% -3.6% to +7.2%
≈ getAABB of a rotated, non default center(180000x) 61.2ms 60.2ms -1.7% -3.6% to +3.1%
≈ getAABB of a non rotated sprite, with custom hitboxes, origin and center(60000x) 19.7ms 19.9ms +0.6% -2.0% to +3.6%
≈ getAABB of a rotated sprite, with custom hitboxes, origin and center(60000x) 67.6ms 67.6ms +0.1% -0.3% to +0.5%
≈ get(400000x) 11.6ms 11.6ms +0.0% -0.3% to +0.5%
≈ Scene: Static sprites (10000) 4.941ms 5.268ms +6.6% -4.8% to +26.1%
≈ Scene: Moving and rotating sprites (5000) 6.460ms 6.568ms +1.7% -6.5% to +16.2%
≈ Scene: Collisions and separation (500 vs 500) 15.8ms 15.3ms -3.2% -8.1% to +3.6%
≈ Scene: Physics 2D (500 bodies) 3.399ms 3.270ms -3.8% -5.7% to +2.6%
≈ Scene: Physics 3D (500 bodies) 12.4ms 13.0ms +4.5% -5.2% to +8.9%
≈ Scene: Platformer characters (300) 1.014ms 0.997ms -1.6% -7.3% to +6.1%
≈ Scene: Text updates (1000) 54.7ms 54.0ms -1.3% -3.3% to +1.7%
≈ Scene: Objects creation and deletion (100 per frame) 1.890ms 1.919ms +1.5% -12.7% to +20.7%
≈ Scene: Heavy events (variables, loops, strings) 1.505ms 1.594ms +5.9% -2.1% to +16.5%

✅ The counters (draw calls, objects count and final state) of the 9 scene benchmarks run on both versions are unchanged.

Comparing 5f89345c to its base 02382da7 (on a pull request, the version tested is the pull request merged into its base), run alternately 4 times on the same machine (AMD EPYC 7763 64-Core Processor, 4 cores, rendering without GPU). Times are the median duration of a batch of iterations (micro benchmarks) or of a frame (scene benchmarks). A change is reported only when larger than 5% and than the differences between the rounds of a same version, and measured by every pair of rounds.

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Physics3D: OOM when stacking scenes (Jolt world not freed on scene pause)

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@Bouh