Add subsection on the problem of slow context switches in kernel-based operating systems.

Documentation/chap-intro.tex

@@ -277,6 +277,27 @@ address space of a single process.  Multics did not have a kernel, but
 it still had the problem of full access to its own address space, so
 that the stack could be overwritten by a defective end-user program.
 
+\subsection{Mediocre input/output performance}
+
+Recent research \cite{Barroso:2017:AKM:3069398.3015146} indicates that
+the performance of input and output in traditional kernel-based
+systems is not good enough for some of the modern devices now becoming
+available.  Recall that, in order to perform some input or output, an
+application program must make a system call so that the kernel can
+perform the operation on behalf of the application.  Things are
+organized this way in order to prevent application programs from
+directly accessing devices so as to protect those devices from getting
+incorrect controls.  Thus, input and output requires a \emph{context
+  switch} which consists of the \emph{system call} itself, a change of
+the \emph{page table} for address translation, and \emph{flushing the
+  cache} since virtual addresses are no longer valid.  Such a context
+switch typically takes around $1 \mu s$.
+
+For typical devices such as disks, performance is not a problem
+because these devices are very slow compared to the time it takes for
+the context switch.  However, for some modern storage devices the slow
+context switch is a problem.
+
 \section{Objectives for a Lisp operating system}
 
 The three main objectives of a Lisp operating system correspond to