Rust concurrency with join and tokio-CodePudding

I am trying to run two functions in parallel with join.

My code is simple:

tokio = { version = "1.14.0", features = ["full"] }

use tokio::join;
use std::thread::sleep;
use std::time::{Duration, Instant};

async fn fn_1() -> i8 {
  sleep(Duration::from_secs(2));

  2
}

async fn fn_2() -> i8 {
  sleep(Duration::from_secs(2));

  1
}

#[tokio::main]
async fn main() -> () {
  let now = Instant::now();

  println!("start: {:#?}", now.elapsed());

  let a = fn_1();
  let b = fn_2();

  join!(a, b);

  println!("end: {:#?}", now.elapsed());
}

But no matter what I do, this takes 4s —2s 2s—, while it should take 2s if I'm not mistaken:

start: 37ns
end: 4.01036111s

Is there something I'm missing?

CodePudding user response：

You're calling the std's sleep functions which put the OS thread to sleep that your program is running on. If you call the tokio::time::sleep functions instead, the futures should be evaluated concurrently.

To enable actual parallelism in execution, you'll need to use tokio::task::spawn to let the runtime decide which thread to run the spawned future on.

For further reading on what blocking is, I recommend this excellent blog post: https://ryhl.io/blog/async-what-is-blocking/