improved futures chapter

book/print.html

@@ -805,6 +805,60 @@ the <code>async</code> and <code>await</code> keywords.</li>
 </ol>
 <p>That's really what Rusts standard library does. As you see there is no definition
 of non-blocking I/O, how these tasks are created or how they're run.</p>
+<h2><a class="header" href="#io-vs-cpu-intensive-tasks" id="io-vs-cpu-intensive-tasks">I/O vs CPU intensive tasks</a></h2>
+<p>As you know now, what you normally write are called non-leaf futures. Let's
+take a look at this async block using pseudo-rust as example:</p>
+<pre><code class="language-rust ignore">let non_leaf = async {
+    let mut stream = TcpStream::connect(&quot;127.0.0.1:3000&quot;).await.unwrap(); &lt;-- yield
+    
+    // request a large dataset
+    let result = stream.write(get_dataset_request).await.unwrap(); &lt;-- yield
+    
+    // wait for the dataset
+    let mut response = vec![];
+    stream.read(&amp;mut response).await.unwrap(); &lt;-- yield
+
+    // do some CPU-intensive analysis on the dataset
+    let report = analyzer::analyze_data(response).unwrap();
+    
+    // send the results back
+    stream.write(report).await.unwrap(); // &lt;-- yield
+};
+</code></pre>
+<p>Now, as you'll see when we go through how Futures work, the code we write between
+the yield points are run on the same thread as our executor.</p>
+<p>That means that while our <code>analyzer</code> is working on the dataset, the executor
+is busy doing calculations instead of handling new requests.</p>
+<p>Fortunately there are a few ways to handle this, and it's not difficult, but it's
+something you must be aware of:</p>
+<ol>
+<li>
+<p>We could create a new leaf future which sends our task to another thread and
+resolves when the task is finished. We could <code>await</code> this leaf-future like any
+other future.</p>
+</li>
+<li>
+<p>The runtime could have some kind of supervisor that monitors how much time
+different tasks take, and move the executor itself to a different thread.</p>
+</li>
+<li>
+<p>You can create a reactor yourself which is compatible with the runtime which
+does the analysis any way you see fit, and returns a Future which can be awaited.</p>
+</li>
+</ol>
+<p>Now #1 is the usual way of handling this, but some executors implement #2 as well.
+The problem with #2 is that if you switch runtime you need to make sure that it
+supports this kind of supervision as well or else you will end up blocking the
+executor.</p>
+<p>#3 is more of theoretical importance, normally you'd be happy by sending the task
+to the thread-pool most runtimes provide.</p>
+<p>Most executors have a way to accomplish #1 using methods like <code>spawn_blocking</code>.</p>
+<p>These methods send the task to a thread-pool created by the runtime where you
+can either perform CPU-intensive tasks or &quot;blocking&quot; tasks which is not supported
+by the runtime.</p>
+<p>Now, armed with this knowledge you are already on a good way for understanding
+Futures, but we're not gonna stop yet, there is lots of details to cover. Take a
+break or a cup of coffe and get ready as we go for a deep dive in the next chapters.</p>
 <h2><a class="header" href="#bonus-section" id="bonus-section">Bonus section</a></h2>
 <p>If you find the concepts of concurrency and async programming confusing in
 general, I know where you're coming from and I have written some resources to