Preface ii
affects the overall performance of the operating system. It is also one of the most
poorly understood and badly documented subsystem in Linux, partially because
there is, quite literally, so much of it. It is very difficult to isolate and understand
individual parts of the code without first having a strong conceptual model of the
whole VM, so this book intends to give a detailed description of what to expect
without before going to the source.
This material should be of prime interest to new developers interested in adapting
the VM to their needs and to readers who simply would like to know how the VM
works. It also will benefit other subsystem developers who want to get the most from
the VM when they interact with it and operating systems researchers looking for
details on how memory management is implemented in a modern operating system.
For others, who are just curious to learn more about a subsystem that is the focus of
so much discussion, they will find an easy to read description of the VM functionality
that covers all the details without the need to plough through source code.
However, it is assumed that the reader has read at least one general operating
system book or one general Linux kernel orientated book and has a general know-
ledge of C before tackling this book. While every effort is made to make the material
approachable, some prior knowledge of general operating systems is assumed.
Book Overview
In chapter 1, we go into detail on how the source code may be managed and de-
ciphered. Three tools will be introduced that are used for the analysis, easy browsing
and management of code. The main tools are the Linux Cross Referencing (LXR)
tool which allows source code to be browsed as a web page and CodeViz for gener-
ating call graphs which was developed while researching this book. The last tool,
PatchSet is for managing kernels and the application of patches. Applying patches
manually can be time consuming and the use of version control software such as
CVS (http://www.cvshome.org/ ) or BitKeeper (http://www.bitmover.com) are not
always an option. With this tool, a simple specification file determines what source
to use, what patches to apply and what kernel configuration to use.
In the subsequent chapters, each part of the Linux VM implementation will be
discussed in detail, such as how memory is described in an architecture independent
manner, how processes manage their memory, how the specific allocators work and
so on. Each will refer to the papers that describe closest the behaviour of Linux
as well as covering in depth the implementation, the functions used and their call
graphs so the reader will have a clear view of how the code is structured. At the
end of each chapter, there will be a “What’s New” section which introduces what to
expect in the 2.6 VM.
The appendices are a code commentary of a significant percentage of the VM. It
gives a line by line description of some of the more complex aspects of the VM. The
style of the VM tends to be reasonably consistent, even between major releases of
the kernel so an in-depth understanding of the 2.4 VM will be an invaluable aid to
understanding the 2.6 kernel when it is released.
