ISBN:0131453483Book Description
Preface Linux is developed with a stronger practical emphasis than a theoretical one. When new algorithms or changes to existing implementations are suggested, it is common to request code to match the argument. Many of the algorithms used in the Virtual Memory (VM) system were designed by theorists, but the implementations have now diverged considerably from the theory. In part, Linux does follow the traditional development cycle of design to implementation, but changes made in reaction to how the system behaved in the "real world" and intuitive decisions by developers are more common. This means that the VM performs well in practice. However, very little VM documentation is available except for a few incomplete overviews on a small number of Web sites, except the Web site containing an earlier draft of this book, of course! This lack of documentation has led to the situation where the VM is fully understood only by a small number of core developers. New developers looking for information on how VM functions are generally told to read the source. Little or no information is available on the theoretical basis for the implementation. This requires that even a casual observer invest a large amount of time reading the code and studying the field of Memory Management. This book gives a detailed tour of the Linux VM as implemented in 2.4.22 and gives a solid introduction of what to expect in 2.6. As well as discussing the implementation, the theory that Linux VM is based on will also be introduced. This is not intended to be a memory management theory book, but understanding why the VM is implemented in a particular fashion is often much simpler if the underlying basis is known in advance. To complement the description, the appendices include a detailed code commentary on a significant percentage of the VM. This should drastically reduce the amount of time a developer or researcher needs to invest in understanding what is happening inside the Linux VM because VM implementations tend to follow similar code patterns even between major versions. This means that, with a solid understanding of the 2.4 VM, the later 2.5 development VMs and the 2.6 final release will be decipherable in a number of weeks. The Intended Audience Anyone interested in how the VM, a core kernel subsystem, works will find answers to many of their questions in this book. The VM, more than any other subsystem, affects the overall performance of the operating system. The VM is also one of the most poorly understood and badly documented subsystems 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 before going to the source. This material should be of prime interest to new developers who want to adapt 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 just want 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 plow 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 knowledge of C before tackling this book. Although 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 deciphered. Three tools are introduced that are used for 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, which was developed while researching this book, for generating call graphs. The last tool, PatchSet, is for managing kernels and the application of patches. Applying patches manually can be time consuming, and using version control software, such as Concurrent Versions Systems (CVS) (http://www.cvshome.org/) or BitKeeper (http://www.bitmover.com), is not always an option. With PatchSet, 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 is 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 chapter will refer to other sources that describe the behavior of Linux, as well as covering in depth the implementation, the functions used and their call graphs so that the reader will have a clear view of how the code is structured. The end of each chapter has 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. They give 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. Companion CD A companion CD is included with this book, and it is highly recommended the reader become familiar with it, especially as you progress more through the book and are using the code commentary. It is recommended that the CD is used with a GNU/Linux system, but it is not required. The text of the book is contained on the CD in HTML, PDF and plain text formats so the reader can perform basic text searches if the index does not have the desired information. If you are reading the .rst edition of the book, you may notice small differences between the CD version and the paper version due to printing deadlines, but the differences are minor. Almost all the tools used to research the book's material are contained on the CD. Each of the tools may be installed on virtually any GNU/Linux installation, references are included to available documentation and the project home sites, so you can check for further updates. With many GNU/Linux installations, there is the additional bonus of being able to run a Web server directly from the CD. The server has been tested with Red Hat 7.3 and Debian Woody but should work with any distribution. The small Web site it provides at http://localhost:10080 offers a number of useful features: A searchable index for functions that have a code commentary available. If a function is searched for that does not have a commentary, the browser will be automatically redirected to LXR. A Web browsable copy of the Linux 2.4.22 source. This allows code to be browsed and identifiers to be searched for. A live version of CodeViz, the tool used to generate call graphs for the book, is available. If you feel that the book's graphs are lacking some detail you want, generate them yourself. The VMRegress, CodeViz and PatchSet packages, which are discussed in Chapter 1, are available in /cdrom/software. gcc-3.0.4 is also provided because it is required for building CodeViz. /p>
Reviews From AMAZON.COM
Doesnt do much more than list and comment on the source code
The book is not well written. If you are looking for a why or a how book this is not it.
Non-trivial subject
A slightly unusual book from Prentice-Hall. If you peruse a list of their recent offerings, this book stands out as rather more specialised. Certainly, in general there is no shortage of linux books, from a variety of publishers. But typically such books are aimed at using linux or they describe applications built atop it. Whereas the technical ability and interest in actually understanding and changing any operating system (not just linux) acts as a tremendous barrier. Hence, the potential audience for Gorman's book is quite exclusive. An upside is that it reduces the potential competition from other authors and publishers.
Gorman is attempting an outreach to potential linux developers. The book has two parts. The first explains key concepts for the VMM. He quickly gets into the issues, with little preamble. In other words, you need at a minimum to be fluent in C, and have some acquaintance with the ideas of memory management, though not necessarily with linux.
Each algorithm in this book is not that intricate. As a rough guide to difficulty level, if you can understand a typical algorithm from the texts by Knuth or Aho or Sedgewick, then you should not have any trouble here.
The second part of the book has code listings with accompanying detailed commentary. This is different from, and substantially improves upon the inline comments, which are sparse to non-existent in the exampled code. To some of you who want to try changing code, the second part's annotations may be the crucial portions of the book. It is rare to see such extensive commentary of source code in book form.