Add optional single-threaded feature to bevy_ecs/bevy_tasks

[james7132]

Objective

Fixes #6689.

Solution

Add single-threaded as an optional non-default feature to bevy_ecs and bevy_tasks that:

• disable the ParallelExecutor as a default runner
• disables the multi-threaded TaskPool
• internally replace QueryParIter::for_each calls with Query::for_each.

Removed the Mutex and Arc usage in the single-threaded task pool.

Future Work/TODO

Create type aliases for Mutex, Arc that change to single-threaaded equivalents where possible.

---

Changelog

Added: Optional default feature multi-theaded to that enables multithreaded parallelism in the engine. Disabling it disables all multithreading in exchange for higher single threaded performance. Does nothing on WASM targets.

[alice-i-cecile]

Okay, I think I'm sold on the feature based approach. It makes this relatively maintainable, and very easy to toggle on and off at a high-level.

[james7132]

There is some open space for discussion on whether we want it to be a feature that is turned on or off to go single threaded. tokio uses rt-multi-thread to enable multi threading and I don't think it's on by default, but we could do that instead. Requiring disabling default-features to switch to single threaded. It'd also make the CI job that runs with --all-features default to what we have now.

[alice-i-cecile]

Conceptually it feels like "multithreading" is a feature we should enable, which should be turned on by default.

We should make sure that both single-threaded and multithreaded paths are being checked in CI (and maybe in miri even): there's a lot of room for subtle mistakes to be introduced.

[recatek]

Could this also allow us to make bevy_tasks an optional dependency for bevy_ecs? The task pools are only used in a few places. Specifically, the par_for_each methods on World (which would be reduced to shims on serial for_each), the parallel executor (which could be #[cfg] disabled entirely) and in benches (which could keep bevy_tasks as a dev-dependency).

[james7132]

We could. However, this would mean that types like ParallelExeuctor straight up don't exist when single threaded is disabled, which can cause compile failures when the feature is enabled. Other parts of the engine will still rely on it since we still need an async executor for async tasks (i.e. asset loading, maybe networking).

[recatek]

Other parts of the engine rely on bevy_tasks or on the parallel executor? If it's the former, could they just pull the parts of bevy_tasks they need out of their own dependency on it?

[Guvante]

This looks in flight but I would note the current version has some sections having a single-threaded feature and others having a default multi-threaded feature, certainly unifying them would be important before merging.

[cart]

This seems like the right approach to me!

[james7132]

Will re-enable once the checks pass.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.

[alice-i-cecile]

I want to review this but the diff looks very messed up. It appears to have incorporated all of the PRs I merged today in the merge train :(

Can you fix / remake this?

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.