Wasm Multi-threading (unstable)

Did you know that Bevy doesn't natively support multi-threaded web apps? The Wasm binary runs all on a single thread, leaving a lot of performance on the table. This can be especially noticeable for audio, resulting in stutters and lag.

The CLI provides an experimental option to build and run apps that use Wasm multi-threading.

Configuration

You can enable support for Wasm multi-threading in Cargo.toml:

[package.metadata.bevy_cli.unstable]
web-multi-threading = true

Alternatively, you can also enable it via the CLI arguments:

bevy run web --unstable multi-threading

What it Does

To be able to use Wasm multi-threading features, the CLI automatically uses the following options:

• Add -C target-feature=+atomics,+bulk-memory to the RUSTFLAGS env variable passed to cargo. This will enable the Wasm features required for multi-threading.
• Add -Z build-std=std,panic_abort to cargo build (only if targeting wasm32-unknown-unknown, wasm32v1-none doesn't need std). This will also enable the Wasm features for std.
• Add the headers Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp to all responses of the web dev server. This enables cross-origin isolation, which is required to use shared memory.

Publishing a Multi-threaded App

It's important to note that your web server must be configured to use cross-origin isolation in order to use Wasm multi-threading. This is a security feature to prevent Spectre-like attacks.

In particular, the server needs to set the Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp headers.

When using itch.io, you can do so by enabling SharedArrayBuffer support. Go to Embed Options > Frame Options > enable SharedArrayBuffer support.