Wasm Multi-Threading (Unstable)
Warning
This feature is unstable and only available when installing the CLI with the unstable feature (enabled by default). Expect to encounter more bugs and a worse user interface until this is stabilized. A nightly Rust toolchain is also required!
Crates such as firewheel-web-audio and bevy_seedling (through its web-audio feature) can take advantage of multi-threading on Wasm. This can be especially beneficial for audio on the web, but requires special flags and setup in order to use. The Bevy CLI can simplify this process for you, making it easier to setup multi-threading for the web.
Note that this does not enable Bevy’s multi-threaded scheduler. The Bevy engine does not yet take advantage of multi-threading on the web, only certain 3rd-party crates do.
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
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.