How to Shrink ammo.js WebAssembly Memory

While WebAssembly does not natively support shrinking its allocated memory buffer during runtime once it has expanded, optimizing and controlling the memory footprint of ammo.js is highly achievable. This article explains why WebAssembly memory cannot be directly shrunk, outlines how to prevent ammo.js memory from growing uncontrollably, and provides practical strategies for reclaiming system resources in web-based physics applications.

The WebAssembly Memory Limitation

WebAssembly (Wasm) utilizes a linear memory model represented as a contiguous array of bytes. In the browser, this is managed via the WebAssembly.Memory object.

During runtime, if ammo.js requires more memory than initially allocated, Emscripten-compiled builds can grow this buffer using the Memory.grow() API (provided ALLOW_MEMORY_GROWTH was enabled during compilation). However, the WebAssembly specification does not currently feature a corresponding shrink instruction. Once the browser allocates a certain buffer size to the Wasm instance, that memory cannot be returned to the host operating system while the instance remains active.

To minimize the memory footprint of ammo.js during runtime, developers must focus on prevention, proper object lifecycle management, and instance recreation.

1. Prevent Growth via Manual Memory Management

Because ammo.js is a direct port of the C++ Bullet Physics engine, JavaScript’s automatic garbage collection does not apply to objects allocated inside the WebAssembly heap. If you create physics bodies, shapes, or vectors without explicitly deleting them, memory leaks will occur, forcing the Wasm heap to grow permanently.

To prevent unnecessary memory expansion: * Always call Ammo.destroy(object) on any temporary or deleted physics objects (such as btVector3, btTransform, btRigidBody, and collision shapes) when they are no longer needed. * Reuse temporary math objects (e.g., vectors and transforms used in render loops) rather than instantiating new ones on every frame.

// Example of reusing a vector to prevent heap growth
const tempVect = new Ammo.btVector3(0, 0, 0);

function updatePhysics(body) {
    tempVect.setValue(0, -9.8, 0); // Reuse existing memory
    body.setGravity(tempVect);
}

2. Re-instantiate ammo.js to Reclaim Memory

If your application transitions between different states—such as loading new levels in a game—and the Wasm heap has grown excessively, the only way to release that memory back to the system is to destroy and recreate the ammo.js instance.

To successfully reclaim memory: 1. Nullify all references to your active ammo.js objects, physics worlds, and the Ammo namespace itself. 2. Allow the browser’s garbage collector to release the underlying WebAssembly.Instance and its associated WebAssembly.Memory buffer. 3. Fetch and initialize a fresh instance of ammo.js for the next session or level.

3. Optimize Initial Emscripten Build Settings

If you compile your own custom build of ammo.js using Emscripten, you can tightly control memory allocations using compilation flags: