### Yielding Control to the Runtime

Recall from the “Our First Async Program” section that at each await point, Rust gives a runtime a chance to pause the task and switch to another one if the future being awaited isn’t ready. The inverse is also true: Rust only pauses async blocks and hands control back to a runtime at an await point. Everything between await points is synchronous.

That means if you do a bunch of work in an async block without an await point, that future will block any other futures from making progress. You may sometimes hear this referred to as one future starving other futures. In some cases, that may not be a big deal. However, if you are doing some kind of expensive setup or long-running work, or if you have a future that will keep doing some particular task indefinitely, you’ll need to think about when and where to hand control back to the runtime.

Let’s simulate a long-running operation to illustrate the starvation problem, then explore how to solve it. Listing 17-14 introduces a `slow` function.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{thread, time::Duration};

    fn main() {
        trpl::block_on(async {
            // We will call `slow` here later
        });
    }

    fn slow(name: &str, ms: u64) {
        thread::sleep(Duration::from_millis(ms));
        println!("'{name}' ran for {ms}ms");
    }

Listing 17-14: Using `thread::sleep` to simulate slow operations

This code uses `std::thread::sleep` instead of `trpl::sleep` so that calling `slow` will block the current thread for some number of milliseconds. We can use `slow` to stand in for real-world operations that are both long-running and blocking.

In Listing 17-15, we use `slow` to emulate doing this kind of CPU-bound work in a pair of futures.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{thread, time::Duration};

    fn main() {
        trpl::block_on(async {
            let a = async {
                println!("'a' started.");
                slow("a", 30);
                slow("a", 10);
                slow("a", 20);
                trpl::sleep(Duration::from_millis(50)).await;
                println!("'a' finished.");
            };

            let b = async {
                println!("'b' started.");
                slow("b", 75);
                slow("b", 10);
                slow("b", 15);
                slow("b", 350);
                trpl::sleep(Duration::from_millis(50)).await;
                println!("'b' finished.");
            };

            trpl::select(a, b).await;
        });
    }

    fn slow(name: &str, ms: u64) {
        thread::sleep(Duration::from_millis(ms));
        println!("'{name}' ran for {ms}ms");
    }

Listing 17-15: Calling the `slow` function to simulate slow operations

Each future hands control back to the runtime only after carrying out a bunch of slow operations. If you run this code, you will see this output:

    'a' started.
    'a' ran for 30ms
    'a' ran for 10ms
    'a' ran for 20ms
    'b' started.
    'b' ran for 75ms
    'b' ran for 10ms
    'b' ran for 15ms
    'b' ran for 350ms
    'a' finished.

As with Listing 17-5 where we used `trpl::select` to race futures fetching two URLs, `select` still finishes as soon as `a` is done. There’s no interleaving between the calls to `slow` in the two futures, though. The `a` future does all of its work until the `trpl::sleep` call is awaited, then the `b` future does all of its work until its own `trpl::sleep` call is awaited, and finally the `a` future completes. To allow both futures to make progress between their slow tasks, we need await points so we can hand control back to the runtime. That means we need something we can await!

We can already see this kind of handoff happening in Listing 17-15: if we removed the `trpl::sleep` at the end of the `a` future, it would complete without the `b` future running at all. Let’s try using the `trpl::sleep` function as a starting point for letting operations switch off making progress, as shown in Listing 17-16.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{thread, time::Duration};

    fn main() {
        trpl::block_on(async {
            let one_ms = Duration::from_millis(1);

            let a = async {
                println!("'a' started.");
                slow("a", 30);
                trpl::sleep(one_ms).await;
                slow("a", 10);
                trpl::sleep(one_ms).await;
                slow("a", 20);
                trpl::sleep(one_ms).await;
                println!("'a' finished.");
            };

            let b = async {
                println!("'b' started.");
                slow("b", 75);
                trpl::sleep(one_ms).await;
                slow("b", 10);
                trpl::sleep(one_ms).await;
                slow("b", 15);
                trpl::sleep(one_ms).await;
                slow("b", 350);
                trpl::sleep(one_ms).await;
                println!("'b' finished.");
            };

            trpl::select(a, b).await;
        });
    }

    fn slow(name: &str, ms: u64) {
        thread::sleep(Duration::from_millis(ms));
        println!("'{name}' ran for {ms}ms");
    }

Listing 17-16: Using `trpl::sleep` to let operations switch off making progress

We’ve added `trpl::sleep` calls with await points between each call to `slow`. Now the two futures’ work is interleaved:

    'a' started.
    'a' ran for 30ms
    'b' started.
    'b' ran for 75ms
    'a' ran for 10ms
    'b' ran for 10ms
    'a' ran for 20ms
    'b' ran for 15ms
    'a' finished.

The `a` future still runs for a bit before handing off control to `b`, because it calls `slow` before ever calling `trpl::sleep`, but after that the futures swap back and forth each time one of them hits an await point. In this case, we have done that after every call to `slow`, but we could break up the work in whatever way makes the most sense to us.

We don’t really want to sleep here, though: we want to make progress as fast as we can. We just need to hand back control to the runtime. We can do that directly, using the `trpl::yield_now` function. In Listing 17-17, we replace all those `trpl::sleep` calls with `trpl::yield_now`.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{thread, time::Duration};

    fn main() {
        trpl::block_on(async {
            let a = async {
                println!("'a' started.");
                slow("a", 30);
                trpl::yield_now().await;
                slow("a", 10);
                trpl::yield_now().await;
                slow("a", 20);
                trpl::yield_now().await;
                println!("'a' finished.");
            };

            let b = async {
                println!("'b' started.");
                slow("b", 75);
                trpl::yield_now().await;
                slow("b", 10);
                trpl::yield_now().await;
                slow("b", 15);
                trpl::yield_now().await;
                slow("b", 350);
                trpl::yield_now().await;
                println!("'b' finished.");
            };

            trpl::select(a, b).await;
        });
    }

    fn slow(name: &str, ms: u64) {
        thread::sleep(Duration::from_millis(ms));
        println!("'{name}' ran for {ms}ms");
    }

Listing 17-17: Using `yield_now` to let operations switch off making progress

This code is both clearer about the actual intent and can be significantly faster than using `sleep`, because timers such as the one used by `sleep` often have limits on how granular they can be. The version of `sleep` we are using, for example, will always sleep for at least a millisecond, even if we pass it a `Duration` of one nanosecond. Again, modern computers are fast: they can do a lot in one millisecond!

This means that async can be useful even for compute-bound tasks, depending on what else your program is doing, because it provides a useful tool for structuring the relationships between different parts of the program (but at a cost of the overhead of the async state machine). This is a form of cooperative multitasking, where each future has the power to determine when it hands over control via await points. Each future therefore also has the responsibility to avoid blocking for too long. In some Rust-based embedded operating systems, this is the only kind of multitasking!

In real-world code, you won’t usually be alternating function calls with await points on every single line, of course. While yielding control in this way is relatively inexpensive, it’s not free. In many cases, trying to break up a compute-bound task might make it significantly slower, so sometimes it’s better for overall performance to let an operation block briefly. Always measure to see what your code’s actual performance bottlenecks are. The underlying dynamic is important to keep in mind, though, if you are seeing a lot of work happening in serial that you expected to happen concurrently!

### Building Our Own Async Abstractions

We can also compose futures together to create new patterns. For example, we can build a `timeout` function with async building blocks we already have. When we’re done, the result will be another building block we could use to create still more async abstractions.

Listing 17-18 shows how we would expect this `timeout` to work with a slow future.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::time::Duration;

    fn main() {
        trpl::block_on(async {
            let slow = async {
                trpl::sleep(Duration::from_secs(5)).await;
                "Finally finished"
            };

            match timeout(slow, Duration::from_secs(2)).await {
                Ok(message) => println!("Succeeded with '{message}'"),
                Err(duration) => {
                    println!("Failed after {} seconds", duration.as_secs())
                }
            }
        });
    }

Listing 17-18: Using our imagined `timeout` to run a slow operation with a time limit

Let’s implement this! To begin, let’s think about the API for `timeout`:

- It needs to be an async function itself so we can await it.
- Its first parameter should be a future to run. We can make it generic to allow it to work with any future.
- Its second parameter will be the maximum time to wait. If we use a `Duration`, that will make it easy to pass along to `trpl::sleep`.
- It should return a `Result`. If the future completes successfully, the `Result` will be `Ok` with the value produced by the future. If the timeout elapses first, the `Result` will be `Err` with the duration that the timeout waited for.

Listing 17-19 shows this declaration.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::time::Duration;

    fn main() {
        trpl::block_on(async {
            let slow = async {
                trpl::sleep(Duration::from_secs(5)).await;
                "Finally finished"
            };

            match timeout(slow, Duration::from_secs(2)).await {
                Ok(message) => println!("Succeeded with '{message}'"),
                Err(duration) => {
                    println!("Failed after {} seconds", duration.as_secs())
                }
            }
        });
    }

    async fn timeout<F: Future>(
        future_to_try: F,
        max_time: Duration,
    ) -> Result<F::Output, Duration> {
        // Here is where our implementation will go!
    }

Listing 17-19: Defining the signature of `timeout`

That satisfies our goals for the types. Now let’s think about the behavior we need: we want to race the future passed in against the duration. We can use `trpl::sleep` to make a timer future from the duration, and use `trpl::select` to run that timer with the future the caller passes in.

In Listing 17-20, we implement `timeout` by matching on the result of awaiting `trpl::select`.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::time::Duration;

    use trpl::Either;

    // --snip--

    fn main() {
        trpl::block_on(async {
            let slow = async {
                trpl::sleep(Duration::from_secs(5)).await;
                "Finally finished"
            };

            match timeout(slow, Duration::from_secs(2)).await {
                Ok(message) => println!("Succeeded with '{message}'"),
                Err(duration) => {
                    println!("Failed after {} seconds", duration.as_secs())
                }
            }
        });
    }

    async fn timeout<F: Future>(
        future_to_try: F,
        max_time: Duration,
    ) -> Result<F::Output, Duration> {
        match trpl::select(future_to_try, trpl::sleep(max_time)).await {
            Either::Left(output) => Ok(output),
            Either::Right(_) => Err(max_time),
        }
    }

Listing 17-20: Defining `timeout` with `select` and `sleep`

The implementation of `trpl::select` is not fair: it always polls arguments in the order in which they are passed (other `select` implementations will randomly choose which argument to poll first). Thus, we pass `future_to_try` to `select` first so it gets a chance to complete even if `max_time` is a very short duration. If `future_to_try` finishes first, `select` will return `Left` with the output from `future_to_try`. If `timer` finishes first, `select` will return `Right` with the timer’s output of `()`.

If the `future_to_try` succeeds and we get a `Left(output)`, we return `Ok(output)`. If the sleep timer elapses instead and we get a `Right(())`, we ignore the `()` with `_` and return `Err(max_time)` instead.

With that, we have a working `timeout` built out of two other async helpers. If we run our code, it will print the failure mode after the timeout:

    Failed after 2 seconds

Because futures compose with other futures, you can build really powerful tools using smaller async building blocks. For example, you can use this same approach to combine timeouts with retries, and in turn use those with operations such as network calls (such as those in Listing 17-5).

In practice, you’ll usually work directly with `async` and `await`, and secondarily with functions such as `select` and macros such as the `join!` macro to control how the outermost futures are executed.

We’ve now seen a number of ways to work with multiple futures at the same time. Up next, we’ll look at how we can work with multiple futures in a sequence over time with streams.