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<h2 id="结合起来看future任务与线程"><a class="header" href="#结合起来看future任务与线程">结合起来看：Future、任务与线程</a></h2>
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<p><a href="https://github.com/rust-lang/book/blob/0d5a0dd395aba1f82d7e5aaf6dbb59b2b843ad2c/src/ch17-06-futures-tasks-threads.md">ch17-06-futures-tasks-threads.md</a></p>
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<p>正如我们在<a href="ch16-00-concurrency.html">第十六章</a>中看到的那样，线程提供了一种实现并发的方式。而在本章里，我们又看到了另一种方式：使用基于 future 和 stream 的 async。如果你想知道应该在什么时候选哪一种，答案是：这取决于具体情况！而且在很多情况下，真正的选择并不是线程<em>或</em> async，而是线程<em>和</em> async 一起用。</p>
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<p>许多操作系统几十年来一直都提供基于线程的并发模型，因此许多编程语言也都支持它们。但这些模型并非没有代价。在许多操作系统中，每个线程都要占用相当可观的内存。线程也只有在你的操作系统和硬件支持它们时才可用。与主流桌面和移动设备不同，一些嵌入式系统甚至根本没有操作系统，因此也就根本没有线程。</p>
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<p>async 模型提供了一组不同的，而且归根结底是互补的权衡。在 async 模型中，并发操作不需要各自独占一个线程。相反，它们可以运行在任务上，就像我们在 stream 那一节里使用 <code>trpl::spawn_task</code> 从同步函数中启动工作时那样。任务和线程有点像，但它不是由操作系统管理，而是由库层面的代码，也就是运行时来管理。</p>
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<p>线程 API 和任务 API 长得这么像并不是偶然。线程是“一组同步操作”的边界；并发发生在线程<em>之间</em>。任务则是“一组异步操作”的边界；并发既可能发生在任务<em>之间</em>，也可能发生在任务<em>内部</em>，因为任务的函数体里可以在多个 future 之间切换。最后，future 是 Rust 中最细粒度的并发单位，而每个 future 又可能代表一棵由其他 future 组成的树。运行时，准确地说是它的 executor，管理任务；任务再去管理 future。从这个角度说，任务有点像“轻量级的、由运行时管理的线程”，只不过由于它们是由运行时而不是操作系统管理，所以又拥有了一些额外能力。</p>
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<p>这并不意味着 async 任务就一定总比线程更好，反过来也一样。在线程基础上的并发，在某些方面其实是比 <code>async</code> 并发更简单的编程模型。这一点既可能是优点，也可能是缺点。线程有点像 “射后不理”（“fire and forget”）：它们没有原生对应 future 的机制，因此通常只是一路运行到结束，除非被操作系统本身打断。</p>
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<p>而线程和任务常常又能很好地协同工作，因为任务（至少在某些运行时里）可以在线程之间来回移动。事实上，我们这一章一直使用的运行时，在底层默认就是多线程的，包括 <code>spawn_blocking</code> 和 <code>spawn_task</code> 在内。许多运行时会采用一种叫作 <em>work stealing</em>（工作窃取）的方法，根据各个线程当前的利用情况，把任务透明地在线程之间迁移，以提高系统整体性能。而这种方法本身就同时需要线程、任务，也就同时需要 future。</p>
|
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<p>在思考什么时候该用哪种方式时，可以先记住这些经验法则：</p>
|
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<ul>
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<li>如果工作是<em>非常适合并行化</em>的，也就是典型的 CPU 密集型任务，比如有一大批数据而且每一部分都能单独处理，那么线程通常是更好的选择。</li>
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<li>如果工作是<em>高度并发</em>的，也就是典型的 I/O 密集型任务，比如要同时处理来自很多不同来源、且到达间隔和频率都各不相同的消息，那么 async 通常是更好的选择。</li>
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</ul>
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<p>如果你同时既需要并行，又需要并发，那也完全不必在“线程”和“async”之间二选一。你可以自由地把它们组合起来，让每一种都去做自己最擅长的那一部分。比如，示例 17-25 展示的就是一种在现实 Rust 代码里相当常见的组合方式。</p>
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<figure class="listing">
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<p><span class="file-name">文件名：src/main.rs</span></p>
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">extern crate trpl; // for mdbook test
|
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</span><span class="boring">
|
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</span>use std::{thread, time::Duration};
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fn main() {
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let (tx, mut rx) = trpl::channel();
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thread::spawn(move || {
|
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for i in 1..11 {
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tx.send(i).unwrap();
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thread::sleep(Duration::from_secs(1));
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}
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});
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trpl::block_on(async {
|
|
|
while let Some(message) = rx.recv().await {
|
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println!("{message}");
|
|
|
}
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});
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}</code></pre>
|
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<figcaption>示例 17-25：在线程中用阻塞代码发送消息，并在 async 代码块中等待这些消息</figcaption>
|
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</figure>
|
|
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<p>我们先创建一个异步信道，然后启动一个线程，用 <code>move</code> 关键字把信道发送端的所有权移入线程中。在那个线程里，我们发送 1 到 10 这些数字，并在每次发送之间睡眠 1 秒。最后，我们像本章前面一直做的那样，把一个由 async 代码块构造出来的 future 交给 <code>trpl::block_on</code> 去执行。在这个 future 里，我们等待那些消息，就像前面那些消息传递示例里做的一样。</p>
|
|
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<p>回到本章一开始举过的场景：想象一下，你用一个专门的线程来执行一组视频编码任务，因为视频编码是计算密集型工作；但当这些操作完成时，再通过一个异步信道通知 UI。现实世界里这类组合的例子多得数不过来。</p>
|
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<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
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<p>这不会是你在本书里最后一次见到并发。<a href="ch21-00-final-project-a-web-server.html">第二十一章</a>中的项目，会把这些概念放到一个比这里那些小例子更真实的场景里来运用，并且更直接地比较“线程”和“任务 / future”这两种解决问题的方式。</p>
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<p>无论你最终选择哪一种方法，Rust 都为你提供了编写安全、快速并发代码所需的工具，不管你的目标是高吞吐的 Web 服务器，还是嵌入式操作系统。</p>
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<p>接下来，我们将讨论：随着 Rust 程序不断变大，应该如何用符合 Rust 惯例的方式来建模问题并组织解决方案。此外，我们也会谈谈 Rust 的这些惯用法，和你可能已经熟悉的面向对象编程风格之间是什么关系。</p>
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