Understanding the Linux Virtual Memory Manager

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 from the theory considerably. In part, Linux does follow the traditional development cycle of design to implementation but it is more common for changes to be made in reaction to how the system behaved in the real-world and intuitive decisions by developers.

This means that the VM performs well in practice but there is very little VM specic documentation available except for a few incomplete overviews in a small number of websites, except the website containing an earlier draft of this book of course! This has lead to the situation where the VM is fully understood only by a small number of core developers. New developers looking for information on how it functions are generally told to read the source and 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 to read the code and study the eld 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 it is is based on will also be introduced. This is not intended to be a memory management theory book but it is often much simpler to understand why the VM is implemented in a particular fashion if the underlying basis is known in advance.

To complement the description, the appendix includes a detailed code commentary on a signicant 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. As 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 nal 2.6 release will be decipherable in a number of weeks.