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A single call to `poll` on a top-level task may potentially do a lot of work before it returns `Poll::Pending`. If a task runs for a long period of time without yielding back to the executor, it can starve other tasks waiting on that executor to execute them, or drive underlying resources. See for example rust-lang/futures-rs#2047, rust-lang/futures-rs#1957, and rust-lang/futures-rs#869. Since Rust does not have a runtime, it is difficult to forcibly preempt a long-running task. Consider a future like this one: ```rust use tokio::stream::StreamExt; async fn drop_all<I: Stream>(input: I) { while let Some(_) = input.next().await {} } ``` It may look harmless, but consider what happens under heavy load if the input stream is _always_ ready. If we spawn `drop_all`, the task will never yield, and will starve other tasks and resources on the same executor. This patch adds a `coop` module that provides an opt-in mechanism for futures to cooperate with the executor to avoid starvation. This alleviates the problem above: ``` use tokio::stream::StreamExt; async fn drop_all<I: Stream>(input: I) { while let Some(_) = input.next().await { tokio::coop::proceed().await; } } ``` The call to [`proceed`] will coordinate with the executor to make sure that every so often control is yielded back to the executor so it can run other tasks. The implementation uses a thread-local counter that simply counts how many "cooperation points" we have passed since the task was first polled. Once the "budget" has been spent, any subsequent points will return `Poll::Pending`, eventually making the top-level task yield. When it finally does yield, the executor resets the budget before running the next task. The budget per task poll is currently hard-coded to 128. Eventually, we may want to make it dynamic as more cooperation points are added. The number 128 was chosen more or less arbitrarily to balance the cost of yielding unnecessarily against the time an executor may be "held up". At the moment, all the tokio leaf futures ("resources") call into coop, but external futures have no way of doing so. We probably want to continue limiting coop points to leaf futures in the future, but may want to also enable third-party leaf futures to cooperate to benefit the ecosystem as a whole. This is reflected in the methods marked as `pub` in `mod coop` (even though the module is only `pub(crate)`). We will likely also eventually want to expose `coop::limit`, which enables sub-executors and manual `impl Future` blocks to avoid one sub-task spending all of their poll budget. Benchmarks (see tokio-rs/tokio#2160) suggest that the overhead of `coop` is marginal.
Tokio
A runtime for writing reliable, asynchronous, and slim applications with the Rust programming language. It is:
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Fast: Tokio's zero-cost abstractions give you bare-metal performance.
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Reliable: Tokio leverages Rust's ownership, type system, and concurrency model to reduce bugs and ensure thread safety.
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Scalable: Tokio has a minimal footprint, and handles backpressure and cancellation naturally.
Website | Guides | API Docs | Roadmap | Chat
Overview
Tokio is an event-driven, non-blocking I/O platform for writing asynchronous applications with the Rust programming language. At a high level, it provides a few major components:
- A multithreaded, work-stealing based task scheduler.
- A reactor backed by the operating system's event queue (epoll, kqueue, IOCP, etc...).
- Asynchronous TCP and UDP sockets.
These components provide the runtime components necessary for building an asynchronous application.
Example
A basic TCP echo server with Tokio:
use tokio::net::TcpListener;
use tokio::prelude::*;
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
let mut 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) => {
eprintln!("failed to read from socket; err = {:?}", e);
return;
}
};
// Write the data back
if let Err(e) = socket.write_all(&buf[0..n]).await {
eprintln!("failed to write to socket; err = {:?}", e);
return;
}
}
});
}
}
More examples can be found here.
Getting Help
First, see if the answer to your question can be found in the Guides or the API documentation. If the answer is not there, there is an active community in the Tokio Discord server. We would be happy to try to answer your question. Last, if that doesn't work, try opening an issue with the question.
Contributing
🎈 Thanks for your help improving the project! We are so happy to have you! We have a contributing guide to help you get involved in the Tokio project.
Related Projects
In addition to the crates in this repository, the Tokio project also maintains several other libraries, including:
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hyper: A fast and correct HTTP/1.1 and HTTP/2 implementation for Rust. -
tonic: A gRPC over HTTP/2 implementation focused on high performance, interoperability, and flexibility. -
warp: A super-easy, composable, web server framework for warp speeds. -
tower: A library of modular and reusable components for building robust networking clients and servers. -
tracing(formerlytokio-trace): A framework for application-level tracing and async-aware diagnostics. -
rdbc: A Rust database connectivity library for MySQL, Postgres and SQLite. -
mio: A low-level, cross-platform abstraction over OS I/O APIs that powerstokio. -
bytes: Utilities for working with bytes, including efficient byte buffers. -
loom: A testing tool for concurrent Rust code
Supported Rust Versions
Tokio is built against the latest stable, nightly, and beta Rust releases. The minimum version supported is the stable release from three months before the current stable release version. For example, if the latest stable Rust is 1.29, the minimum version supported is 1.26. The current Tokio version is not guaranteed to build on Rust versions earlier than the minimum supported version.
License
This project is licensed under the MIT license.
Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tokio by you, shall be licensed as MIT, without any additional terms or conditions.