The Evolution of Complexity in Apple Darwin: A Common Coupling Point of View

Common coupling increases the interdependencies between software modules. It should be avoided if possible. In previous work, we presented two types of categorization of common coupling, one is for single-kernel-based software, one is for multi-kernel-based-software. In this paper, we analyze the relationships between these two types of categorization and apply them to study the evolution of the complexity of Apple Darwin. The same conclusion about Darwin’s evolution is drawn based on the two types of categorization of common coupling: From version XNU-517 to version XNU-792, Darwin has restructured to reduce the number of difficulty-inducing high category (level) global variables in order to reduce the system complexity. However, due to the definition-use dependencies, the complexity of Darwin induced by global variables has increased from version XNU-517 to version XNU-792. 1

Maintainability of the kernels of open-source operating systems : a comparison of Linux with FreeBSD, NetBSD, and OpenBSD

We compared and contrasted the maintainability of four open-source operating systems: Linux, FreeBSD, NetBSD, and OpenBSD. We used our categorization of common coupling in kernel-based software to highlight future maintenance problems. An unsafe definition is a definition of a global variable that can affect a kernel module if that definition is changed. For each operating system we determined a number of measures, including the number of global variables, the number of instances of global variables in the kernel and overall, as well as the number of unsafe definitions in the kernel and overall. We also computed the value of each our measures per kernel KLOC and per KLOC overall. For every measure and every ratio, Linux compared unfavorably with FreeBSD, NetBSD, and OpenBSD. Accordingly, we are concerned about the future maintainability of Linux.9 page(s