Understanding the Linux® Virtual Memory Manager
By Mel Gorman
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.
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.
