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.

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.

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