Dissecting Code Features: An Evolutionary Analysis of Kernel Versus Nonkernel Code in Operating Systems

Understanding the evolution of software systems is crucial for advancing software engineering practices. Many studies have been devoted to exploring software evolution. However, they primarily treat software as an entire entity and overlook the inherent differences between subsystems, which may lead to biased conclusions. In this study, we attempt to explore variations between subsystems by investigating the code feature differences between kernel and nonkernel components from an evolutionary perspective. Based on three operating systems as case studies, we examine multiple dimensions, including the code churn characteristics and code inherent characteristics. The main findings are as follows: (1) The proportion of kernel code remains relatively small, and exhibits consistent stability across the majority of versions as systems evolve. (2) Kernel code exhibits higher stability in contrast to nonkernel code, characterized by a lower modification rate and finer modification granularity. The patterns of modification activities are similar in both kernel and nonkernel code, with a preference of changing code and a tendency to avoid the combination of adding and deleting code. (3) The cumulative code size and complexity of kernel files show an upward trajectory as the system evolves. (4) Kernel files exhibit a significantly higher code density and complexity than nonkernel files, featuring a greater number of code line, comments, and statements, along with a larger program length, vocabulary, and volume. Conversely, kernel functions prioritize modularity and maintainability, with a significantly smaller size and lower complexity than nonkernel functions. These insights contribute to a deeper understanding of the dynamics within operating system codebases and highlight the necessity of targeted maintenance strategies for different subsystems.