Object Oriented Discrete-Event Simulation CS4730 Fall 2010 Jose M. Garrido Department of Computer...

Object Oriented Discrete-Event Simulation

CS4730 Fall 2010

Jose M. Garrido

Department of Computer Science and Information Systems

Kennesaw State University

Technical Attractions of Technical Attractions of SimulationSimulation**

Ability to compress time, expand time Ability to control sources of variation Avoids errors in measurement Ability to stop and review Ability to restore system state Facilitates replication Modeler can control level of detail

*Discrete-Event Simulation: Modeling, Programming, and Analysis by G. Fishman, 2001, pp. 26-27

2(C) J. M. Garrido, 2009

IntroductionIntroduction

What is discrete-event simulation? Modeling, simulating, and analyzing systems Computational and mathematical techniques

Model: construct a conceptual framework that describes a system

Simulate: perform experiments using computer implementation of the model

Analyze: draw conclusions from output that assist in decision making process

We will first focus on the model


Characterizing a ModelCharacterizing a Model

Deterministic or Stochastic Does the model contain stochastic components? Randomness is easy to add to a DES

Static or Dynamic Is time a significant variable?

Continuous or Discrete Does the system state evolve continuously or only at

discrete points in time? Continuous: classical mechanics Discrete: queuing, inventory, machine shop models


DefinitionsDefinitions

Discrete-Event Simulation Model Stochastic: some state variables are random Dynamic: time evolution is important Discrete-Event: significant changes occur at

discrete time instances Monte Carlo Simulation Model

Stochastic Static: time evolution is not important


Programming LanguagesProgramming Languages

General-purpose programming languages Flexible and familiar Well suited for learning DES principles and

techniques E.g.: C, C++, Java, Eiffel, Fortran, Ada

Special-purpose simulation languages Most commercial simulation products are IDEs and

not programming languages Good for building models quickly Provide built-in features (e.g., queue structures) Most provide graphics and animation E.g.: Arena, Flexim, Promodel, others 6(C) J. M. Garrido, 2009

TerminologyTerminology

Model vs. Simulation (noun) Model can be used WRT conceptual,

specification, or computational levels Simulation is rarely used to describe the

conceptual or specification model Simulation is frequently used to refer to the

computational model (program) Model vs. Simulate (verb)

To model can refer to development at any of the levels

To simulate refers to computational activity Meaning should be obvious from the

context 7(C) J. M. Garrido, 2009

ObservationsObservations

Make each model as simple as possible Never simpler Do not ignore relevant characteristics Do not include extraneous characteristics

Model development is not sequential Steps are often iterated In a team setting, some steps will be in parallel Do not merge verification and validation

Develop models at three levels Do not jump immediately to computational level Think a little, program a lot (and poorly);

Think a lot, program a little (and well)8(C) J. M. Garrido, 2009

Final Goal

Discrete Event Simulation can provide valuable insight about the system


Using SimulationUsing Simulation

Begin by studying trace-driven single server queue

Follow that with a trace-driven machine shop model


Approaches to Discrete-Event Simulation

Event approach Activity approach Process interaction approach

Object oriented Psim3, and PsimJ2 use the full power of

process interaction simulation. Other simulation packages and languages

have limited OO support11(C) J. M. Garrido, 2009

The Process Interaction Approach

A simulation model using this approach consists of: A definition of the active and passive

entities in the model. A description of how different active

entities are going to interact among themselves, as time passes.

A simulation run consists of creating and starting the passive and active objects.


Advantages of the Process Style of Simulation

Compatible with the Object-Oriented approach to modeling and programming, every process is an active object.

Suitable for modeling large and complex systems.

Threads are used to implement the active objects in the simulation models.


Process Interaction Approach to Simulation with PsimJ2

A portable library package of Java classes Object-oriented Supports the process interaction approach A good tool for courses, such as:

Object-oriented Simulation Object-oriented design and programming Operating systems Introduction to real-time systems


Applying the Process Interaction Approach to Simulation

The major components in a model are entities, these are modeled as active objects

The active entities have a life of their own, and are called processes

The passive entities are resources and queues

The UML Collaboration diagram shows the process interactions


Developing Simulation Models of Real-Time Systems

The simulation models are more complex because they need to satisfy the requirements of real-time systems.

A set of tasks are identified in developing such systems.

Identifying these tasks will simplify the

analysis and design phases of building a simulation model of real-time systems.


Developing Models of RTS

Concurrency – Multiple simultaneous activities. Synchronous Communication – Two or more

communicate, where the sender will wait until the receiver gets the message.

Timing Constraints – Describe the time windows during which activities must occur or respond.

Communications Deadlines – Timers are used to place deadlines on how long a process can wait to communicate with another process.


Train-Gate System Example

Train-Gate System consists of a one-directional railway track crossing a road.

The gate at the crossing must be lowered or raised by a Controller system.

Sensors are used to detect entering trains and leaving trains.

The signals from the sensors are sent to a Monitor, which keeps track of the number of trains in the area.


Case Study:Train-Gate System

Train Entry Sensor Train Exit Sensor

MonitorController Gate

Open

CloseOpen/Close Signals

Gate Opened / Closed

Discrete Interarrival Times of Trains

This simulation model is available at:http:cience.kennesaw.edu/~jgarrido/train


Train-Gate System • The real-time behavior of the Train-Gate System is

modeled using Communicating Real-Time State Machines (CRSM).

• The timing constraints in CRSM are specified in the transitions between states, as shown in the diagram.

• The system design includes the common set of tasks identified in previous slides.

• Simulation of the Train-Gate system has been implemented using PsimJ2, a Java based discrete-event simulation package.

• The implementation of the system defines the processes: Controller, Gate, Train, Monitor, and Sensors.


(C) J Garrido 21

Processes

Processes are implemented as threads in Java.

The main body of a process is composed of a sequence of phases, after a phase is executed the control is returned to the simulation executive.

The process executing a phase is the currently running process.

From the system point of view, all processes are active and interacting at the same time.

(C) J Garrido 22

Process View

Static Process Description Dynamic Process Management

Initiate Process Delay (Time) Process Suspend Process Activate Process Terminate Process

(C) J Garrido 23

Resource View

Standard/Tailored Resource Handler Resource Management

Create resource Request resource Acquire resource Release resource Status of resource Destroy resource

(C) J Garrido 24

Simulation Executive for the Process Interaction Approach

Schedule processes at particular instants, i.e., place processes in the event list

Remove processes from the event list ( to make them idle or to terminate them)

Re-schedule processes, i.e., change position of processes in the event list.

Execute the phase for the “running” process.

(C) J Garrido 25

The Simulation Executive

(C) J Garrido 26

Java Threads

A thread consists of a set of phases, each of which is executed when the simulation executive advances the clock to the required time.

Every thread re-starts execution at its own reactivation point (in the active phase), and its corresponding event time.

(C) J Garrido 27

Thread With Its Reactivation Points

(C) J Garrido 28

Using PsimJ2

Every simulation model defines a set of classes that will be instantiated appropriately

The classes that represent processes need to inherit class Process, which is a thread class.

The other classes defined are used to create passive objects.

(C) J Garrido 29

Active and Passive Objects

(C) J. M. Garrido, 2009 30

Questions?

Object Oriented Discrete-Event Simulation CS4730 Fall 2010 Jose M. Garrido Department of Computer...

Documents

Transcript of Object Oriented Discrete-Event Simulation CS4730 Fall 2010 Jose M. Garrido Department of Computer...