Entities and Objects The major components in a model are entities, entity types are implemented as Java classes The active entities have a life of their own, and are called processes The passive entities are resources and queues (C) J. M. Garrido

Objects in a Simulation Model There are several objects in a simulation model The activate objects are instances of the classes that inherit the library class Process The passive objects are instances of any other class, including some of the library classes (e.g., queue classes) (C) J. M. Garrido

Single-Server Models There is only one server object that provides service to customer objects Arriving customer objects join a queue to wait for service The random events are customer arrivals and service completions (C) J. M. Garrido

Simple Queue A waiting line for arriving customers (or other objects). It is a passive entity in the model, used as ‘global’ shared resource. An arriving customer can only enter the queue at the tail A customer can only leave if it is at the head of the queue The usual order of customers is FCFS. (C) J. M. Garrido

A Simple Queue (C) J. M. Garrido

The Car-Wash Model Cars arrive for complete wash (service) Arriving cars join a line (queue) to wait for service There is only one car-wash machine that can service one car at a time After the service is complete for a car, it leaves the system (C) J. M. Garrido

The Carwash Model Components A set of customer processes, the cars One server process, the wash-machine One conceptual process that models arrivals (this represents the environment) One global shared resource, the queue. Events: customer arrival start of service end of service and departure of a customer (C) J. M. Garrido

Single Server System Model (C) J. M. Garrido

The Conceptual Model The model of the simple carwash system consists of the following processes: The Car process, which define the customer objects The Arrivals process, which represent the environment The Wash-machine process, which represents the server. (C) J. M. Garrido

Modeling Diagrams The UML diagram shows the main entities in the model The UML static modeling diagrams show the basic structure of the classes and their relationships The dynamic modeling diagrams show all the process interactions and the use of resources for every model. (C) J. M. Garrido

UML class diagrams for Car and Wash-machine (C) J. M. Garrido

UML Class Diagram for the Carwash Model (C) J. M. Garrido

UML Collaboration Diagram for the Carwash Model (C) J. M. Garrido

UML Sequence Diagram for the Carwash Model (C) J. M. Garrido

UML State Diagram for a Car Object (C) J. M. Garrido

Results of a Simulation Run The trace, which is a sequence of all relevant events with time of occurrence The summary results (statistics), with the values of the performance metrics of the model for the current run (C) J. M. Garrido

Performance Measures in the Carwash System The average number of customer in the system The average number of customers in the queue(s) (i.e., that are waiting) The average time that a customer spends in the system The average time that a customer spends in the queue(s) The server (wash-machine) utilization. (C) J. M. Garrido

Performance and Workload Characterization The usual objective for determining the performance measures in a queuing system is to achieve the following criteria: Reduce the customer waiting periods Improve the server utilization Maximize throughput (the number of customers served) for a given workload. (C) J. M. Garrido

Workload Parameters The performance of the system depends on the workload submitted, and for this queuing model it consists of the following parameters: The average customer arrival rate,  The average customer service rate,  The resource demand for the customers (C) J. M. Garrido

System Parameters The queue size The resource capacity of the system. (C) J. M. Garrido

Performance Measures Depend on Workload The performance metrics computed in the single-server model, depend on the workload submitted, and on the system parameters. Modifying the workload on a model and/or the system parameters changes the behavior of the model. (C) J. M. Garrido

Examples of Performance and Workload To compare two models with different servers, is equivalent to changing the workload by providing two different values for the average service time demands of the customers. If the system workload increases, the server utilization will also increase (C) J. M. Garrido

Bottleneck The bottleneck of the system at capacity will be localized in the server or resource with a utilization of 1, while the other servers or resources each have utilization significantly below 1. The bottleneck can be localized at the server, the queue, or at the resources. (C) J. M. Garrido

Studying System Behavior All the changes that occur in the system are analyzed In the model of the simple car-wash system, the system changes state when a customer arrives, when a customer starts service, when a customer completes service, etc. These changes are instantaneous occurrences and are called events. (C) J. M. Garrido

The Car-wash Stochastic Model These models include entities with attributes that change value in a non-deterministic manner The occurrence of the random events follow a probabilistic distribution (C) J. M. Garrido

Random Variables in the Model In the Car-wash model, the following random variables are defined: The inter-arrival period for the car objects The service period for each car object. (C) J. M. Garrido

Generation of Random Variables To facilitate the simulation of random events, PsimJ provides random number generators using several probability distributions Some of the distributions return an integer (long) value, others return a real (double) value (C) J. M. Garrido

Probability Distributions in PsimJ Uniform Exponential Poisson Normal Erlang Geometric Hyper-exponential (C) J. M. Garrido

Random Events in the Carwash Model Two random events: customer arrivals end of a customer service Two random variables are modeled: inter-arrival periods service periods Both random variables follow an exponential distribution. (C) J. M. Garrido

Documentation on PsimJ2 The sample models included in the PsimJ2 Web page http://science.kennesaw.edu/~jgarrido/psim.html (C) J. M. Garrido

Using PsimJ2 with Java All processes (active objects) are created from a user-defined class that inherits the library class Process. All non-process objects (passive objects) are created directly from the corresponding library class (e.g., class Squeue), or other classes. (C) J. M. Garrido

The Structure of a Model in Java Define all classes for the model’s processes, and inherit from the library class Process. Define one class (main class) that includes method main. In method Main_body of the main class, start the simulation by invoking method start_sim, then calculate summary statistics. (C) J. M. Garrido

Method main In method main: Define a simulation object of the library class Simulation; give a name to the model. Create all the objects used in the model. Start the active objects. See Listing 5.2 (C) J. M. Garrido

Using GUI with Models Java facilitates the implementation of graphical interfaces for simulation models A general GUI framework is included with PsimJ2. The package name is gui. To run the carwash model with GUI, run the CarwashGUI class, instead of the Carwash class. (C) J. M. Garrido

Carwash Simulation Run (C) J. M. Garrido

Carwash Simulation Run (2) (C) J. M. Garrido