Ch. 17: pull disconnected material out of 17.01

Author: chriskrycho

src/ch17-01-tasks.md

@@ -1,24 +1,11 @@
 ## Futures and the Async Syntax
 
-As we saw in the previous chapter, threads provide one approach to concurrency,
-and they let us solve some of these issues. However, they also have some
-tradeoffs. On many operating systems, they use a fair bit of memory for each
-thread, and they come with some overhead for starting up and shutting down.
-Threads are also only an option when your operating system and hardware support
-them! While mainstream desktop and mobile operating systems have all had
-threading for many years, many embedded operating systems, like those used on
-some microcontrollers, do not.
-
-The async model provides a different—and ultimately complementary—set of
-tradeoffs. In
-
-<!-- TODO: the following paragraph is not where it needs to be structurally. -->
-
-In the async model, concurrent operations do not require their own threads.
-Instead, they can run on *tasks*. A task is a bit like a thread, but instead of
-being managed by the operating system, it is managed by a runtime.
-
-<!-- TODO: connective tissue as it were. -->
+<!--
+  TODO: connective tissue as it were. Also, an open question on whether we want
+  to use “task” as the primary term here. Futures do not actually *require* a
+  task primitive to run (and there are runtimes which do *not* use tasks!) so it
+  might make more sense to find another common, appropriate term for it instead.
+-->
 
 Like other languages with async, Rust uses the `async` and `await`
 keywords—though with some important differences, as we will see. Blocks and

src/ch17-03-vs-threads.md

@@ -0,0 +1,16 @@
+<!-- TODO: find the right home for this (maybe nowhere!) -->
+
+As we saw in the previous chapter, threads provide one approach to concurrency,
+and they let us solve some of these issues. However, they also have some
+tradeoffs. On many operating systems, they use a fair bit of memory for each
+thread, and they come with some overhead for starting up and shutting down.
+Threads are also only an option when your operating system and hardware support
+them! While mainstream desktop and mobile operating systems have all had
+threading for many years, many embedded operating systems, like those used on
+some microcontrollers, do not.
+
+The async model provides a different—and ultimately complementary—set of
+tradeoffs. In the async model, concurrent operations do not require their own
+threads. Instead, they can run on *tasks*. A task is a bit like a thread, but
+instead of being managed by the operating system, it is managed by code that
+lives at the level of libraries.