已有epoll、libuv、Mio等非阻塞IO，是否仍需Rust的async/await？

Great question—since you already have hands-on experience with epoll and libuv, this should click once we clarify the relationship between these tools. Let’s break this down:

Do you need async/await when using Mio or raw epoll? #

Short answer: No, you don’t.

Mio is essentially a safe, Rust-native abstraction over epoll (and other OS-specific IO multiplexers like kqueue on macOS). Just like with epoll or libuv, you can build high-performance single/multi-threaded servers using Mio by:

• Setting up an event loop
• Registering file descriptors/sockets with interest in specific events (read, write, etc.)
• Polling for ready events in a loop
• Handling each event as it fires

This is a purely event-driven, synchronous programming model—no async/await required. You’re in full control of the event loop and how events are dispatched, just like you would be with raw epoll.

So what’s async/await actually for? #

Your confusion about async/await’s file read/write examples makes total sense at first glance—but let’s clear up a key misunderstanding: async/await isn’t a replacement for event-driven IO; it’s a higher-level abstraction built on top of it.

When you write code like tokio::fs::read_to_string("file.txt").await, you’re not blocking the thread. Under the hood, the async runtime (like Tokio or async-std) is using tools like Mio or even io_uring to handle non-blocking IO. The .await syntax doesn’t “wait” in a blocking way—it tells the runtime:

“Pause this task right here, and go run other ready tasks while we wait for this IO operation to complete. Come back to me once the data is ready.”

The runtime manages the event loop, task scheduling, and IO event notifications for you. This lets you write asynchronous code that looks linear (no messy callbacks or state machines), while still getting the performance benefits of non-blocking IO.

Async/await shines in these scenarios: #

• Simplifying complex asynchronous workflows: If you have a connection that needs to handle a sequence of operations (e.g., read a request → query a database → write a response), async/await lets you write this as linear code instead of manually managing state transitions.
• Composing multiple async operations: Need to fetch data from 3 different APIs at once and process all results when they’re done? Tools like join! or select! make this trivial, without having to wire up multiple event registrations and callbacks.
• Leveraging the Rust async ecosystem: Most modern Rust libraries for network IO, databases, and HTTP (like reqwest, sqlx, tonic) are built around async/await. Using these libraries lets you build production-ready apps quickly without reinventing the wheel.
• Reducing boilerplate: Writing a custom event loop with Mio works, but it’s tedious for most applications. Async runtimes handle all the low-level details (like thread pooling, task scheduling, and event handling) so you can focus on your business logic.

A quick recap #

• Mio/raw epoll: Low-level tools for full control over event-driven IO. Use these when you need fine-grained control over the event loop, or when building a runtime yourself.
• Async/await: High-level abstraction built on top of non-blocking IO. Use these when you want to write clean, maintainable asynchronous code without dealing with the nitty-gritty of event loops.

You can even use them together if you need to—many async runtimes (like Tokio) use Mio under the hood! But for most applications, async/await will be the more productive choice, while Mio is better for cases where you need to customize the event-driven behavior.

内容的提问来源于stack exchange，提问作者neevek