Chapter 9: Petri Nets

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9.1 Introduction

Every tool applied to the modeling and analysis of computer systems has its place. Petri nets have a place in computer systems performance assessment, ranging somewhere between analytical queuing theory and computer simulation. This is due to the nature of Petri nets and their ability to model concurrency, synchronization, mutual exclusion, conflict, and system state more completely than analytical models but not as completely as simulations. They have a fundamental theory dictating their analysis, but they act more like simulations in that they allow the modeler to examine single entities within the system, as well as their movement and effect on the state of the entire system.

Petri nets (PNs) provide a graphical tool as well as a notational method for the formal specification of systems. The systems they model tend to include more than simply an arrival rate and a service rate. They are used in situations where each entity passing through the system can bring individual state information, which can be used to more completely and accurately model complex interactions such as contention and concurrency.

Petri nets were first introduced in 1966 to describe concurrent systems. This initial introduction was followed by continual improvements-for example, the addition of timing to transitions, priority to transitions, types to tokens, and colors depicting conditions on places and tokens. These have been followed by the development of software tools to aid in the modeling and analysis of systems using Petri net concepts.

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