Is Kernel Code Different From Non-Kernel Code? A Case Study of BSD Family Operating Systems

TL;DR

This study investigates the evolutionary characteristics of kernel and non-kernel code in BSD operating systems, revealing significant differences in commit size and review times at the subsystem level that are not captured by global metrics.

Contribution

It provides an empirical analysis of code churn and review dynamics at the subsystem level, highlighting important differences between kernel and non-kernel code in BSD OSes.

Findings

Kernel commits are larger than non-kernel commits.

Code review times are longer for kernel code.

Most commits contain either kernel or non-kernel code exclusively.

Abstract

Code churn and code velocity describe the evolution of a code base. Current research quantifies and studies code churn and velocity at a high level of abstraction, often at the overall project level or even at the level of an entire company. We argue that such an approach ignores noticeable differences among the subsystems of large projects. We conducted an exploratory study on four BSD family operating systems: DragonFlyBSD, FreeBSD, NetBSD, and OpenBSD. We mine 797,879 commits to characterize code churn in terms of the annual growth rate, commit types, change type ratio, and size taxonomy of commits for different subsystems (kernel, non-kernel, and mixed). We also investigate differences among various code review periods, i.e., time-to-first-response, time-to-accept, and time-to-merge, as indicators of code velocity. Our study provides empirical evidence that quantifiable evolutionary code characteristics at a global system scope fail to take into account significant individual differences that exist at a subsystem level. We found that while there exist similarities in the code base growth rate and distribution of commit types (neutral, additive, and subtractive) across BSD subsystems, (a) most commits contain kernel or non-kernel code exclusively, (b) kernel commits are larger than non-kernel commits, and (c) code reviews for kernel code take longer than non-kernel code.