Researchers have been studying the quality attributes of operating system code for more than two decades [Henry and Kafura 1981], [Yu et al. 2004]. Particularly close to the work you’re reading here are comparative studies of open source operating systems [Yu et al. 2006], [Izurieta and Bieman 2006], and studies comparing open and closed source systems [Stamelos et al. 2002], [Paulson et al. 2004], [Samoladas et al. 2004].

A comparison of maintainability attributes between the Linux and various Berkeley Software Distribution (BSD) operating systems found that Linux contained more instances of module communication through global variables (known as common coupling) than the BSD variants. The results I report here corroborate this finding for file-scoped identifiers, but not for global identifiers (see Figure 15-11). Furthermore, an evaluation of growth dynamics of the FreeBSD and Linux operating systems found that both grow at a linear rate, and that claims of open source systems growing at a faster rate than commercial systems are unfounded [Izurieta and Bieman 2006].

A study by Paulson and his colleagues [Paulson et al. 2004] compares evolutionary patterns between three open source projects (Linux, GCC, and Apache) and three non-disclosed commercial ones. They found a faster rate of bug fixing and feature addition in the open source projects, which is something we would expect for very popular projects like those they examine. In another study focusing on the quality of the code (its internal quality attributes) [Stamelos et al. 2002] the authors used a commercial tool to evaluate 100 open source applications using metrics similar to those reported here, but measured on a scale ranging from accept to rewrite. They then compared the results against benchmarks supplied by the tool’s vendor for commercial projects. The authors found that only half of the modules they examined would be considered acceptable by software organizations applying programming standards based on software metrics. A related study by the same group [Samoladas et al. 2004] examined the evolution of a measure called maintainability index [Coleman et al. 1994] between an open source application and its (semi)proprietary forks. They concluded that all projects suffered from a similar deterioration of the maintainability index over time.