Module tokio::runtime

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The Tokio runtime.

Unlike other Rust programs, asynchronous applications require runtime support. In particular, the following runtime services are necessary:

  • An I/O event loop, called the driver, which drives I/O resources and dispatches I/O events to tasks that depend on them.
  • A scheduler to execute tasks that use these I/O resources.
  • A timer for scheduling work to run after a set period of time.

Tokio’s Runtime bundles all of these services as a single type, allowing them to be started, shut down, and configured together. However, often it is not required to configure a Runtime manually, and a user may just use the tokio::main attribute macro, which creates a Runtime under the hood.

§Usage

When no fine tuning is required, the tokio::main attribute macro can be used.

use tokio::net::TcpListener;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let listener = TcpListener::bind("127.0.0.1:8080").await?;

    loop {
        let (mut socket, _) = listener.accept().await?;

        tokio::spawn(async move {
            let mut buf = [0; 1024];

            // In a loop, read data from the socket and write the data back.
            loop {
                let n = match socket.read(&mut buf).await {
                    // socket closed
                    Ok(n) if n == 0 => return,
                    Ok(n) => n,
                    Err(e) => {
                        println!("failed to read from socket; err = {:?}", e);
                        return;
                    }
                };

                // Write the data back
                if let Err(e) = socket.write_all(&buf[0..n]).await {
                    println!("failed to write to socket; err = {:?}", e);
                    return;
                }
            }
        });
    }
}

From within the context of the runtime, additional tasks are spawned using the tokio::spawn function. Futures spawned using this function will be executed on the same thread pool used by the Runtime.

A Runtime instance can also be used directly.

use tokio::net::TcpListener;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::runtime::Runtime;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Create the runtime
    let rt  = Runtime::new()?;

    // Spawn the root task
    rt.block_on(async {
        let listener = TcpListener::bind("127.0.0.1:8080").await?;

        loop {
            let (mut socket, _) = listener.accept().await?;

            tokio::spawn(async move {
                let mut buf = [0; 1024];

                // In a loop, read data from the socket and write the data back.
                loop {
                    let n = match socket.read(&mut buf).await {
                        // socket closed
                        Ok(n) if n == 0 => return,
                        Ok(n) => n,
                        Err(e) => {
                            println!("failed to read from socket; err = {:?}", e);
                            return;
                        }
                    };

                    // Write the data back
                    if let Err(e) = socket.write_all(&buf[0..n]).await {
                        println!("failed to write to socket; err = {:?}", e);
                        return;
                    }
                }
            });
        }
    })
}

§Runtime Configurations

Tokio provides multiple task scheduling strategies, suitable for different applications. The runtime builder or #[tokio::main] attribute may be used to select which scheduler to use.

§Multi-Thread Scheduler

The multi-thread scheduler executes futures on a thread pool, using a work-stealing strategy. By default, it will start a worker thread for each CPU core available on the system. This tends to be the ideal configuration for most applications. The multi-thread scheduler requires the rt-multi-thread feature flag, and is selected by default:

use tokio::runtime;

let threaded_rt = runtime::Runtime::new()?;

Most applications should use the multi-thread scheduler, except in some niche use-cases, such as when running only a single thread is required.

§Current-Thread Scheduler

The current-thread scheduler provides a single-threaded future executor. All tasks will be created and executed on the current thread. This requires the rt feature flag.

use tokio::runtime;

let rt = runtime::Builder::new_current_thread()
    .build()?;
§Resource drivers

When configuring a runtime by hand, no resource drivers are enabled by default. In this case, attempting to use networking types or time types will fail. In order to enable these types, the resource drivers must be enabled. This is done with Builder::enable_io and Builder::enable_time. As a shorthand, Builder::enable_all enables both resource drivers.

§Lifetime of spawned threads

The runtime may spawn threads depending on its configuration and usage. The multi-thread scheduler spawns threads to schedule tasks and for spawn_blocking calls.

While the Runtime is active, threads may shut down after periods of being idle. Once Runtime is dropped, all runtime threads have usually been terminated, but in the presence of unstoppable spawned work are not guaranteed to have been terminated. See the struct level documentation for more details.

§Detailed runtime behavior

This section gives more details into how the Tokio runtime will schedule tasks for execution.

At its most basic level, a runtime has a collection of tasks that need to be scheduled. It will repeatedly remove a task from that collection and schedule it (by calling poll). When the collection is empty, the thread will go to sleep until a task is added to the collection.

However, the above is not sufficient to guarantee a well-behaved runtime. For example, the runtime might have a single task that is always ready to be scheduled, and schedule that task every time. This is a problem because it starves other tasks by not scheduling them. To solve this, Tokio provides the following fairness guarantee:

If the total number of tasks does not grow without bound, and no task is blocking the thread, then it is guaranteed that tasks are scheduled fairly.

Or, more formally:

Under the following two assumptions:

  • There is some number MAX_TASKS such that the total number of tasks on the runtime at any specific point in time never exceeds MAX_TASKS.
  • There is some number MAX_SCHEDULE such that calling poll on any task spawned on the runtime returns within MAX_SCHEDULE time units.

Then, there is some number MAX_DELAY such that when a task is woken, it will be scheduled by the runtime within MAX_DELAY time units.

(Here, MAX_TASKS and MAX_SCHEDULE can be any number and the user of the runtime may choose them. The MAX_DELAY number is controlled by the runtime, and depends on the value of MAX_TASKS and MAX_SCHEDULE.)

Other than the above fairness guarantee, there is no guarantee about the order in which tasks are scheduled. There is also no guarantee that the runtime is equally fair to all tasks. For example, if the runtime has two tasks A and B that are both ready, then the runtime may schedule A five times before it schedules B. This is the case even if A yields using yield_now. All that is guaranteed is that it will schedule B eventually.

Normally, tasks are scheduled only if they have been woken by calling wake on their waker. However, this is not guaranteed, and Tokio may schedule tasks that have not been woken under some circumstances. This is called a spurious wakeup.

§IO and timers

Beyond just scheduling tasks, the runtime must also manage IO resources and timers. It does this by periodically checking whether there are any IO resources or timers that are ready, and waking the relevant task so that it will be scheduled.

These checks are performed periodically between scheduling tasks. Under the same assumptions as the previous fairness guarantee, Tokio guarantees that it will wake tasks with an IO or timer event within some maximum number of time units.

§Current thread runtime (behavior at the time of writing)

This section describes how the current thread runtime behaves today. This behavior may change in future versions of Tokio.

The current thread runtime maintains two FIFO queues of tasks that are ready to be scheduled: the global queue and the local queue. The runtime will prefer to choose the next task to schedule from the local queue, and will only pick a task from the global queue if the local queue is empty, or if it has picked a task from the local queue 31 times in a row. The number 31 can be changed using the global_queue_interval setting.

The runtime will check for new IO or timer events whenever there are no tasks ready to be scheduled, or when it has scheduled 61 tasks in a row. The number 61 may be changed using the event_interval setting.

When a task is woken from within a task running on the runtime, then the woken task is added directly to the local queue. Otherwise, the task is added to the global queue. The current thread runtime does not use the lifo slot optimization.

§Multi threaded runtime (behavior at the time of writing)

This section describes how the multi thread runtime behaves today. This behavior may change in future versions of Tokio.

A multi thread runtime has a fixed number of worker threads, which are all created on startup. The multi thread runtime maintains one global queue, and a local queue for each worker thread. The local queue of a worker thread can fit at most 256 tasks. If more than 256 tasks are added to the local queue, then half of them are moved to the global queue to make space.

The runtime will prefer to choose the next task to schedule from the local queue, and will only pick a task from the global queue if the local queue is empty, or if it has picked a task from the local queue global_queue_interval times in a row. If the value of global_queue_interval is not explicitly set using the runtime builder, then the runtime will dynamically compute it using a heuristic that targets 10ms intervals between each check of the global queue (based on the worker_mean_poll_time metric).

If both the local queue and global queue is empty, then the worker thread will attempt to steal tasks from the local queue of another worker thread. Stealing is done by moving half of the tasks in one local queue to another local queue.

The runtime will check for new IO or timer events whenever there are no tasks ready to be scheduled, or when it has scheduled 61 tasks in a row. The number 61 may be changed using the event_interval setting.

The multi thread runtime uses the lifo slot optimization: Whenever a task wakes up another task, the other task is added to the worker thread’s lifo slot instead of being added to a queue. If there was already a task in the lifo slot when this happened, then the lifo slot is replaced, and the task that used to be in the lifo slot is placed in the thread’s local queue. When the runtime finishes scheduling a task, it will schedule the task in the lifo slot immediately, if any. When the lifo slot is used, the coop budget is not reset. Furthermore, if a worker thread uses the lifo slot three times in a row, it is temporarily disabled until the worker thread has scheduled a task that didn’t come from the lifo slot. The lifo slot can be disabled using the disable_lifo_slot setting. The lifo slot is separate from the local queue, so other worker threads cannot steal the task in the lifo slot.

When a task is woken from a thread that is not a worker thread, then the task is placed in the global queue.

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