Introduce the two main sections by a paragraph in the introduction.

Documentation/chap-garbage-collection.tex

@@ -16,6 +16,12 @@ used for a file system in secondary memory.
 There is a rich literature on automatic memory management.
 (see e.g., \cite{Jones:2011:GCH:2025255})
 
+For \sysname{}, we plan to have a two-level memory management
+technique.  The low level consists of a relatively small heap for each
+thread, and a per-thread garbage collector that manages that heap.
+The higher-level contains long-lived objects and objects that are
+shared between several threads.
+
 \section{Per-thread garbage collector}
 
 Each thread has a local heap, roughly the size of the cache, say