单线程阻塞

  1. fn get_two_sites() {
  2.     // Handle two task in a row.
  3.     download("https://www.foo.com");
  4.     download("https://www.bar.com");
  5. }

单线程方式必须执行完一个任务才能执行下一个，一个系统线程也会造成长时间阻塞，总任务处理时间长。

多线程阻塞

  1. fn get_two_sites() {
  2.     // Spawn two threads to do work.
  3.     let thread_one = thread::spawn(|| download("https://www.foo.com"));
  4.     let thread_two = thread::spawn(|| download("https://www.bar.com"));
  6.     // Wait for both threads to complete.
  7.     thread_one.join().expect("thread one panicked");
  8.     thread_two.join().expect("thread two panicked");
  9. }

开启独立的线程分别执行两个任务，总的执行时长取决于执行慢的任务，并发程度高，但开启大量线程会增加系统上下文开销。

异步

Rust 中将任务抽象成 Future ，在执行每个 Future 时，如果发生阻塞，就会让出当前的线程控制权，允许其它 future 运行，这样可以极大的利用操作系统的线程。

  1. async fn get_two_sites_async() {
  2.     // Create two different "futures" which, when run to completion,
  3.     // will asynchronously download the webpages.
  4.     let future_one = download_async("https://www.foo.com");
  5.     let future_two = download_async("https://www.bar.com");
  7.     // Run both futures to completion at the same time.
  8.     join!(future_one, future_two);
  9. }