A Comparison of OS Extension Technologies

The current trend in operating systems research is to allow applications to dynamically extend the kernel to improve application performance or extend functionality, but the most effective approach to extensibility remains unclear. Some systems use safe languages to permit code to be downloaded directly into the kernel; other systems provide in-kernel interpreters to execute extension code; still others use software techniques to ensure the safety of kernel extensions. The key characteristics that distinguish these systems are the philosophy behind extensibility and the technology used to implement extensibility. This paper presents a taxonomy of the types of extensions that might be desirable in an extensible operating system, evaluates the performance cost of various extension technologies currently being employed, and compares the cost of adding a kernel extension to the benefit of having the extension in the kernel. Our results show that compiled technologies (e.g. Modula-3 and software fault isolation) are good candidates for implementing general-purpose kernel extensions, but that the overhead of interpreted languages is sufficiently high that they are inappropriate for this use.