rust - How to create a dedicated threadpool for CPU-intensive work in Tokio?

Question

I have a Rust async server based on the Tokio runtime. It has to process a mix of latency-sensitive I/O-bound requests, and heavy CPU-bound requests.

I don't want to let the CPU-bound tasks monopolize the Tokio runtime and starve the I/O bound tasks, so I'd like to offload the CPU-bound tasks to a dedicated, isolated threadpool (isolation is the key here, so spawn_blocking/block_in_place on one shared threadpool are insufficient). How can I create such a threadpool in Tokio?

A naive approach of starting two runtimes runs into an error:

thread 'tokio-runtime-worker' panicked at 'Cannot start a runtime from within a runtime. This happens because a function (like block_on) attempted to block the current thread while the thread is being used to drive asynchronous tasks.'

use tokio; // 0.2.20

fn main() {
    let mut main_runtime = tokio::runtime::Runtime::new().unwrap();
    let cpu_pool = tokio::runtime::Builder::new().threaded_scheduler().build().unwrap();
    let cpu_pool = cpu_pool.handle().clone(); // this is the fix/workaround!

    main_runtime.block_on(main_runtime.spawn(async move {
        cpu_pool.spawn(async {}).await
    }))
    .unwrap().unwrap();
}

Can Tokio allow two separate runtimes? Is there a better way to create an isolated CPU pool in Tokio?

Answer 1

While Tokio already has a threadpool, the documentation of Tokio advises:

If your code is CPU-bound and you wish to limit the number of threads used to run it, you should run it on another thread pool such as rayon. You can use an oneshot channel to send the result back to Tokio when the rayon task finishes.

So, if you want to create a threadpool to make heavy use of CPU, a good way is to use a crate like Rayon and send the result back to the Tokio task.