Chapter 26 Extensible systems

Objectives

To examine alternative OS structures beyond monolithic kernels and to introduce the kinds of problem tackled by current OS research

Points to emphasise

• A common theme for Scout, SPIN, Nemesis and Exokernel designs was the desire to support multi-media applications. These can require substantial processing resources, soft-real-time scheduling and sometimes support for developing new network protocols. Such concerns led to mechanisms for tailoring the system to a particular workload, or for providing more careful resource management than with UNIX.

• Programming language run-time environments such as the JVM perform many of the same functions as a traditional operating system.

Possible difficulties

• The systems presented here are generally prototype ones produced by university research projects. Aside from Mach, they have not often been developed to the same level of completion as the UNIX systems of Chapters 25, 26.

Teaching hints

• Current commodity systems are capable of handling audio and video data at a respectable quality. Discuss this in relation to the motivation for specialized OS designs for soft-real-time systems. Is it simply that computing hardware has become so fast that there is no need to take special care over resource management?
• Discuss the advantages and disadvantages of allowing individual applications lower-level control over the environment in which they execute – for instance as Nemesis and Exokernel systems provide by using unprivileged library operating systems. Two major points are the trade-off between intra-application and inter-application optimization of resource usage and ensuring that common resources are accessed correctly (for instance the representation of meta-data on disk or the implementation of network protocols).
• Beyond the systems discussed here, examine the facilities provided by other operating systems, for example Epoch, VxWorks or LynxOS and discuss how those system are tailored to their environment. A topic for more general discussion is whether there could ever be a single OS that is acceptable across all hardware devices and, if so, how it would use the ideas from these extensible operating systems.