Comparing Rust Async Runtimes:
Tokio vs. async-std

Last updated: April 20, 2025

1. Introduction: The Need for Async Runtimes

Rust's async/.await syntax provides language-level support for writing asynchronous code, which is crucial for building efficient I/O-bound applications like web servers and network clients. However, the language itself doesn't dictate *how* these asynchronous tasks (represented by Futures) are executed.

This execution is handled by an async runtime. The runtime is responsible for polling Futures, scheduling tasks, handling I/O events (like network readiness), and managing timers. Several runtimes exist in the Rust ecosystem, with Tokio and async-std historically being the most prominent.

This article compares Tokio and async-std, examining their design goals, features, ecosystem impact, and current status to help you understand the landscape. We'll also briefly mention smol as a relevant alternative.

For background on async programming in Rust, see our Writing Concurrent Applications in Rust guide.

2. What Does an Async Runtime Do?

An async runtime typically provides:

• Executor/Scheduler: Decides which tasks (Futures) to run and when. Manages waking up tasks when they are ready (e.g., after an I/O operation completes or a timer fires).
• Reactor/I/O Driver: Interfaces with the operating system's event notification system (like epoll, kqueue, IOCP) to efficiently handle non-blocking I/O operations.
• Timers: Provides functionality for tasks to sleep or wait for specific durations.
• Task Spawning: Allows creating new asynchronous tasks that run concurrently.
• Synchronization Primitives: Often includes async versions of mutexes, channels, etc.

3. Tokio: The De Facto Standard

3.1 Philosophy & Features

Tokio is the most widely used and feature-rich async runtime in Rust. Its philosophy emphasizes performance, reliability, and providing a comprehensive toolkit for building network applications.

• Feature-Rich: Offers extensive built-in support for async TCP/UDP, timers, file system operations, process management, synchronization primitives, and more.
• Performance-Focused: Highly optimized for speed and low overhead, leveraging zero-cost abstractions where possible.
• Ecosystem Centric: Many foundational crates in the async web ecosystem (like Hyper, Tonic, Axum, Reqwest) are built on or tightly integrated with Tokio.
• Explicit Runtime Management: Requires users to explicitly start and configure the runtime, often using the #[tokio::main] or #[tokio::test] macros or manually using tokio::runtime::Builder.

3.2 Scheduler

Tokio provides two main scheduler options:

• Multi-Threaded Scheduler (Default): A work-stealing scheduler designed to utilize multiple CPU cores efficiently for high-throughput applications. Tasks can be stolen and executed by different worker threads.
• Current-Thread Scheduler: A lighter-weight scheduler that runs all tasks on the thread where the runtime was started. Useful for scenarios where multi-threading is not needed or desired.

3.3 Ecosystem

Tokio's biggest strength is its dominant position in the ecosystem. If you plan to use popular web frameworks (Axum, Actix-web often uses Tokio features), gRPC libraries (Tonic), or HTTP clients (Reqwest), you will likely need to use the Tokio runtime, as these libraries often depend on Tokio's specific I/O types and reactor.

4. async-std: The Standard Library Mirror (Status Update)

4.1 Philosophy & Features

async-std emerged with the goal of providing an async runtime whose API closely mirrors Rust's standard library (std). The idea was to make the transition from synchronous to asynchronous code feel familiar.

• Std-like API: Provided async versions of many std types and functions (e.g., async_std::fs, async_std::net::TcpStream, async_std::task::spawn).
• Simplicity Goal: Aimed for a simpler setup and API compared to Tokio's initial complexity.
• Implicit Runtime Start (Historically): Unlike Tokio's explicit setup, async-std often started its runtime implicitly when the first async operation requiring it (like I/O or timers) was called. This was sometimes seen as convenient but could also lead to surprises or conflicts if mixed with other runtimes.
• Work-Stealing Scheduler: Also employed a multi-threaded work-stealing scheduler.

4.2 Current Status (Discontinued)

While initially a strong alternative, development velocity on async-std slowed significantly. As of early 2025, the project has been officially discontinued. The README on its GitHub repository now directs users towards the smol runtime instead.

Due to its discontinued status and Tokio's ecosystem dominance, starting new projects with async-std is not recommended. However, understanding its history and goals provides context for the evolution of async Rust.

5. Smol: A Lighter Alternative

Smol is positioned as a simpler, smaller, and more modular async runtime. It aims to be easier to understand and integrate. While less feature-rich out-of-the-box compared to Tokio, it offers a viable alternative for those seeking simplicity or wanting to avoid the full Tokio framework dependency. It's the recommended alternative for former async-std users.

6. Key Differences Summarized

7. Choosing a Runtime

For most new projects, especially those involving web servers, gRPC, or common networking libraries, Tokio is the pragmatic choice due to ecosystem compatibility. Its performance is excellent, and while it has many features, the basic usage via #[tokio::main] is straightforward.

Consider Smol if:

• You prioritize simplicity and a smaller dependency footprint.
• You are not relying heavily on libraries that mandate Tokio.
• You are migrating from async-std.

Avoid starting new projects with async-std.

8. Conclusion

The Rust async ecosystem has largely consolidated around Tokio as the primary runtime, driven by its feature set, performance, and adoption by key libraries. While async-std played an important role in exploring API design, its development has ceased.

Tokio offers a robust and powerful platform for building asynchronous applications, and for simpler needs or those avoiding the wider Tokio ecosystem, Smol presents a lean alternative. Understanding the role of the runtime and the practical implications of ecosystem compatibility is key when working with async Rust.

9. Additional Resources

Related Articles

• Writing Concurrent Applications in Rust
• Common Rust Crates for Web Development
• Rust vs. Go: Performance and Concurrency Comparison

External Links

• Tokio Official Website & Docs
• async-std Repository (Discontinued)
• Smol Repository
• The Async Rust Book
• Blog Post: The State of Async Rust: Runtimes (Provides good context)