How to Increase Emscripten Memory Limit in Ammo.js

When building complex 3D physics simulations using ammo.js, you may encounter out-of-memory errors due to Emscripten’s default memory limits. This article provides a straightforward guide on how to increase the default memory allocation and enable dynamic memory growth in your ammo.js application using Emscripten compilation flags.

Understanding the Limit

By default, Emscripten allocates a static memory size (typically 16MB) for the WebAssembly (Wasm) heap. If your ammo.js physics world contains thousands of rigid bodies, complex collision meshes, or dense heightmaps, the simulation will quickly exhaust this space, resulting in a “Cannot enlarge memory arrays” or “Out of memory” crash.

To resolve this, you must adjust the memory settings when compiling ammo.js from its source code.

Step 1: Increase the Initial Memory Limit

To set a larger fixed memory size, you need to rebuild ammo.js using the -s INITIAL_MEMORY compiler flag.

The value passed to this flag must be specified in bytes and must be a multiple of 65,536 (64 KB), which is the WebAssembly page size.

For example, to increase the initial memory to 128MB (134,217,728 bytes), add the following flag to your Emscripten compilation command:

-s INITIAL_MEMORY=134217728

To set it to 256MB (268,435,456 bytes):

-s INITIAL_MEMORY=268435456

Step 2: Enable Dynamic Memory Growth (Recommended)

If you do not know the exact memory requirements of your application beforehand, you can allow the memory heap to grow dynamically at runtime. This prevents crashes if your physics world expands unexpectedly.

To enable this, use the -s ALLOW_MEMORY_GROWTH=1 flag during compilation:

-s ALLOW_MEMORY_GROWTH=1

Note: While dynamic memory growth is highly convenient, it can introduce minor performance overhead when the memory expands, as the JavaScript engine must reallocate the WebAssembly memory buffer.

Step 3: Applying the Flags to the Ammo.js Build Command

When building ammo.js using the Emscripten toolchain (usually via a terminal or a build script like cmake or make), append these flags to the emcc command.

A typical compilation command incorporating both a 128MB initial limit and dynamic growth looks like this:

emcc ammo.idl -o ammo.js -s INITIAL_MEMORY=134217728 -s ALLOW_MEMORY_GROWTH=1 -s NO_EXIT_RUNTIME=1 -O3

By rebuilding ammo.js with these parameters, your application will have access to the necessary memory resources required to run complex, large-scale physics simulations smoothly.