Summary

In this chapter, we have looked at programs in which threads both start and terminate dynamically during program execution. We have shown that it is possible to construct satisfactory finite state models for this sort of program by using a static population of processes with cyclic behavior. The model fixes the maximum number of concurrently active processes while concurrent thread activation in the program is only limited by available storage. We regard a model as satisfactory if it exhibits the same behavior as the program it is modeling with respect to safety and liveness properties. We were able to use the golf club model to investigate and correct a liveness problem and subsequently to implement that correction in the program.

The main example concerned a resource allocator that managed a pool of identical reusable resources. Threads competed for access to these resources. In our example, the resources were golf balls. In computing systems, this form of allocation is required for resources such as memory pages and message buffers.

The results of this chapter show that the models we constructed in previous chapters for resource access also apply to programs in which threads are created dynamically. For example, the Readers – Writers program of Chapter 7 had a fixed number of Reader and Writer threads with cyclic behavior. However, the monitor that controlled read/write access would work equally well in a program with a dynamically varying number of Reader and Writer threads.

Finally, this chapter demonstrated the use of the Java join() method which allows one thread to await the termination of another. The example was a master–slave arrangement in which the master thread obtained the result of the slave computation.