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Runtime architecture

So far we've been talking about async runtimes as an abstract concept. Let's dig a bit deeper into the way they are implemented—as you'll see soon enough, it has an impact on our code.

Flavors

tokio ships two different runtime flavors.

You can configure your runtime via tokio::runtime::Builder:

Builder::new_multi_thread gives you a multithreaded tokio runtime
Builder::new_current_thread will instead rely on the current thread for execution.

#[tokio::main] returns a multithreaded runtime by default, while #[tokio::test] uses a current thread runtime out of the box.

Current thread runtime

The current-thread runtime, as the name implies, relies exclusively on the OS thread it was launched on to schedule and execute tasks.
When using the current-thread runtime, you have concurrency but no parallelism: asynchronous tasks will be interleaved, but there will always be at most one task running at any given time.

Multithreaded runtime

When using the multithreaded runtime, instead, there can up to N tasks running in parallel at any given time, where N is the number of threads used by the runtime. By default, N matches the number of available CPU cores.

There's more: tokio performs work-stealing.
If a thread is idle, it won't wait around: it'll try to find a new task that's ready for execution, either from a global queue or by stealing it from the local queue of another thread.
Work-stealing can have significant performance benefits, especially on tail latencies, whenever your application is dealing with workloads that are not perfectly balanced across threads.

Implications

tokio::spawn is flavor-agnostic: it'll work no matter if you're running on the multithreaded or current-thread runtime. The downside is that the signature assume the worst case (i.e. multithreaded) and is constrained accordingly:

pub fn spawn<F>(future: F) -> JoinHandle<F::Output>
where
    F: Future + Send + 'static,
    F::Output: Send + 'static,
{ /* */ }

Let's ignore the Future trait for now to focus on the rest.
spawn is asking all its inputs to be Send and have a 'static lifetime.

The 'static constraint follows the same rationale of the 'static constraint on std::thread::spawn: the spawned task may outlive the context it was spawned from, therefore it shouldn't depend on any local data that may be de-allocated after the spawning context is destroyed.

fn spawner() {
    let v = vec![1, 2, 3];
    // This won't work, since `&v` doesn't
    // live long enough.
    tokio::spawn(async { 
        for x in &v {
            println!("{x}")
        }
    })
}

Send, on the other hand, is a direct consequence of tokio's work-stealing strategy: a task that was spawned on thread A may end up being moved to thread B if that's idle, thus requiring a Send bound since we're crossing thread boundaries.

fn spawner(input: Rc<u64>) {
    // This won't work either, because
    // `Rc` isn't `Send`.
    tokio::spawn(async move {
        println!("{}", input);
    })
}

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