Chapter 9: Dynamic Systems

Overview

In the programs we have described so far, threads are created during initialization and they run until program termination. The organization of threads, and the monitors through which they interact in these programs, has been static during program execution. This chapter examines systems in which thread creation and termination occur dynamically during program execution. The number of active threads thus varies as execution progresses. This sort of behavior occurs in operating systems where processes are created dynamically in response to user commands. For example, when a user clicks on the word processor icon, a process is created to run the word processor program; when the user exits the word processor program, the process terminates.

To illustrate some of the issues involved in modeling and programming dynamic systems, we consider the problem of resource allocation in which dynamically created threads need variable amounts of a resource to proceed. To make the problem less abstract, we simulate a golf club with players simulated by threads and golf balls representing the resources they require.

A specific problem addressed in this chapter is the relevance of finite state models to dynamic systems. Briefly stated, the problem is that the models we construct must be static with a fixed number of processes to permit analysis, while a dynamic system has a variable number of processes. To see how this is handled, we proceed by developing the program first and then developing a model. We examine how much of the behavior of the dynamic system is captured in the static model and whether this static model is helpful in analyzing the behavior of the dynamic system.

Finally, the chapter looks at the use of the Java join() method, which permits a thread to wait for the termination of another thread, which it has usually created dynamically.