a little progress

book.tex

@@ -3849,22 +3849,53 @@ must therefore perform automatic garbage collection.
 \section{Garbage Collection}
 \label{sec:GC}
 
+Here we study a relatively simple algorithm for garbage collection
+that is the basis of the state-of-the-art generational garbage
+collectors~\citep{Lieberman:1983aa,Ungar:1984aa,Jones:1996aa,Detlefs:2004aa,Dybvig:2006aa}. In
+particular, we describe a two-space copying
+collector~\citep{Wilson:1992fk} that uses Cheney's algorithm to
+perform the
+copy~\citep{Cheney:1970aa}. Figure~\ref{fig:copying-collector} gives a
+coarse-grained depiction of what happens in a two-space collector,
+showing two time steps, prior to garbage collection on the top and
+after garbage collection on the bottom. In a two-space collector, the
+heap is segmented into two parts, the FromSpace and the
+ToSpace. Initial, all allocations go to the FromSpace. As you can see,
+prior to garbage collection all of the allocated objects are in the
+FromSpace.
+
+A running program has direct access to registers and the procedure
+call stack, and those may contain pointers into the heap. Those
+pointers are called the \emph{root set}. In
+Figure~\ref{fig:copying-collector} there are three pointers in the
+root set, one in a register and two on the stack. The goal of the
+garbage collector is to 1) preserve all objects that are reachable
+from the root set via a path of pointers, i.e., the \emph{live}
+objects and 2) reclaim the storage of everything else, i.e., the
+\emph{garbage}. A copying collector accomplished this by copying all
+of the live objects into the ToSpace and then performs a slight of
+hand, treating the ToSpace as the new FromSpace and the old FromSpace
+as the new ToSpace. In the bottom of
+Figure~\ref{fig:copying-collector} you can see the result of the copy.
+All of the live objects have been copied to the ToSpace in such a way
+that preserves the pointer relationships. For example, the pointer in
+the register still points to a 2-tuple whose first element is a
+3-tuple and second element is a 2-tuple.
 
 \begin{figure}[tbp]
-\includegraphics[width=0.5\textwidth]{CopyingCollector}
-\includegraphics[width=0.5\textwidth]{CopyCollector2}
+\centering
+\includegraphics[width=0.9\textwidth]{CopyingCollector} \\
+\includegraphics[width=0.9\textwidth]{CopyCollector2}
 \caption{A copying collector in action.}
 \label{fig:copying-collector}
 \end{figure}
 
-\cite{Wilson:1992fk}
 
-Figure~\ref{fig:copying-collector}
 
 
 
-\section{Integrating Garbage Collection}
-\label{sec:integratng-GC}
+\section{Compiler Integration}
+\label{sec:compiler-integration}