Function spawn_balanced 

pub fn spawn_balanced<F>(future: F) -> JoinHandle<F::Output>
where
    F: Future + Send + 'static,
    F::Output: Send + 'static,

Spawn a task on one of the local runtimes, with a work balance heuristic.

Use this when you can do your work on any runtime and it does not matter which. Be mindful of cross-runtime synchronization and await, which is more costly in a level runtime than a regular tokio multi-thread runtime.