PART IV CASE STUDIES

Chapter 24 Classical UNIX

Objectives

To understand the structure and behaviour of the UNIX kernel. To criticise the basic design and to study how more recent UNIX systems have met these criticisms.

Points to emphasise

• UNIX is about thirty years old; although the foundations remain the same, a lot of what we see in current UNIX-derived systems was not present in the original design.
• There are good and bad aspects of the basic UNIX design. Chapter 25 shows how UNIX has been adapted for multi-processor machines and for networked computers.
• IPC was probably the worst aspect of the original design. System V has rewritten IPC. It offers shared memory with semaphores and local message passing. BSD developed networking which subsumes pipes.
• UNIX is executed procedurally (in-process).
• The UNIX process model is heavyweight (one process per address space).
• System calls are synchronous and potentially blocking.
• It is difficult to build systems above classical UNIX. We can’t have the multithreading we need. This is bad for both system services (e.g. communications packages such as RPC, see Chapter 13) and higher level applications.
• The UNIX interface has become a de-facto standard. More recent systems are constrained to support the UNIX interface so that existing software will still run on them.

Possible difficulties

The material should be easy to understand since we have often used UNIX as an example.

Procedural execution of the UNIX kernel can make some of the detail difficult to understand. How does bottom-up I/O handling get done? What happens when a process goes to sleep?

Teaching hints

• Avoid a flat description. Take a critical approach. UNIX is so successful it can take it! Why is it so successful? Are other commercial systems even worse?
• If the students can use UNIX systems set up practical work to illustrate command composition, dynamic process creation, pipelines of processes and so on.