The new async-std runtime

inspired by Go

Posted by hughnian on December 18, 2019

Stop worrying about blocking: the new async-std runtime, inspired by Go

async-std is a mature and stable port of the Rust standard library to its new async/await world, designed to make async programming easy, efficient, worry- and error-free.

We announced async-std on August 16th - exactly 4 month ago. Our focus for the initial release was providing a stable and reliable API for users to build async applications on, modelled after Rust standard library. It came with a number of innovative implementations: The first implementation of a JoinHandle based task API and single-allocation tasks.

Today, we’re introducing the new async-std runtime. It features a lot of improvements, but the main news is that it eliminates a major source of bugs and performance issues in concurrent programs: accidental blocking.

In summary:

  • The new runtime is really fast and outperforms the old one.
  • The new runtime is universal in the sense that it adapts to different workloads automatically, becoming single-threaded or multi-threaded on demand. If a single thread can handle all the work, we don’t pay the price of work-stealing.
  • The new runtime is conceptually simpler, by removing the split between the non-blocking and the blocking threadpool.
  • The new runtime detects blocking automatically. We don’t need spawn_blocking anymore and can simply deprecate it.
  • The new runtime makes blocking efficient. Rather than always paying the cost of spawn_blocking, we only offload blocking work to a separate thread if the work really blocks.

The new task scheduling algorithm and the blocking strategy are adaptations of ideas used by the Go runtime to Rust.

The problem of blocking

Until today, a constant source of frustration when writing async programs in Rust was the possibility of accidentally blocking the executor thread when running a task. To avoid that problem, async-std used to provide the unstable spawn_blocking function that moves potentially blocking work onto a special thread pool so that the executor thread can keep making progress.

However, spawn_blocking is not a bulletproof solution because we have to remember to invoke it manually every time we expect to block. But it’s very difficult to even reliably predict what kind of code can block. Programmers have to carefully separate async and blocking code, which is an infamous problem discussed by the blog post titled What Color Is Your Function.

Separation of code gets even harder when you consider that operations that may block the current executor thread include plain expensive computations, such as transforming an image or sorting lots of data. Conversely, sometimes we’ll pessimistically assume some code blocks while in practice it may be very quick, which means we’re paying the price of spawn_blocking even when don’t have to.

With the new async-std runtime, such difficulties become a thing of the past: should a task execute for too long, the runtime will automatically react by spawning a new executor thread taking over the current thread’s work. This strategy eliminates the need for separating async and blocking code.

A concrete example

To illustrate what all this means in practice, let’s use std::fs::read_to_string as an example of a blocking operation. The async version of it used to be implemented as follows:

async fn read_to_string<P: AsRef<Path>>(path: P) -> io::Result<String> {
    let path = path.as_ref().to_owned();
    spawn_blocking(move || std::fs::read_to_string(path)).await
Note two crucial things:
  • We call spawn_blocking to isolate the blocking operation.
  • We always clone the path and execute the blocking operation on a separate threadpool, even if the file is cached and will be read in an instant.

The new runtime relieves you of these concerns and allows you to do the blocking operation directly inside the async function:

async fn read_to_string(path: impl AsRef<Path>) -> io::Result<String> {

The runtime measures the time it takes to perform the blocking operation and if it takes a while, a new thread is automatically spawned and replaces the old executor thread. That way, only the current task is blocked on the operation rather than the whole executor thread or the whole runtime. If the blocking operation is quick, we don’t spawn a new thread and therefore no additional cost is inflicted.

If you still want to make sure the operation runs in the background and doesn’t block the current task either, you can simply spawn a regular task and do the blocking work inside it:

async fn read_to_string(path: impl AsRef<Path>) -> io::Result<String> {
    let path = path.as_ref().to_owned();
    spawn(async move { std::fs::read_to_string(path) }).await

Note the use of spawn instead of spawn_blocking.

Web frameworks commonly need asynchronous I/O while performing substantial work per request. The new runtime enables you to fearlessly use synchronous libraries like diesel or rayon in any async-std application.


In our initial tests, the new scheduler performs better then our old one, while still staying small and well understandable.

The following benchmark was run on two EC2 instances . A minihttp server based on the old runtime (master branch) and the new runtime (new-scheduler branch) runs on an m5a.8xlarge instance, while wrk (a benchmark tool) runs on a separate m5a.16xlarge instance.

The benchmark has three different scenarios with different arguments passed to wrk. Option -t configures the number of threads, -c configures the number of TCP connections, and -d configures the duration of benchmark in seconds. See the readme for more details on how to run the benchmark.

Small and well-documented

The new runtime is small, uses no unsafe and is documented. Please take a look at the source to see how it works. Feel free to ask questions on the pull request! There are still plenty of optimization opportunities and we will continue to blog about those details!

Trying it out

To try out the new scheduler before release, modify your Cargo.toml this way:

async-std = { git = '', branch = 'new-scheduler' }

Please report on your experiences - and report potential bugs!


The new async-std runtime relieves programmers of the burden of isolating blocking code from async code. You simply don’t have to worry about it anymore.

The adaptive nature of the new runtime allows it to use less resources when multithreading does not bring any benefit. This improves performance in CLI tools and lowers the latency in web servers. At the same time, the runtime will scale up to use all available resources during intense workloads.

All these changes make it easier to write async programs while also making them more efficient and reliable!

We would like to thank all contributors to async-std, big and small, new and long-term and all the library authors building great stuff for the Rust async ecosystem!