Discrete-event simulation: Difference between revisions

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{{Short description|Type of simulation}}

A '''discrete-event simulation''' ('''DES''') models the operation of a [[system]] as a ([[discrete time|discrete]]) [[sequence of events]] in time. Each event occurs at a particular instant in time and marks a change of [[State (computer science)|state]] in the system.<ref>{{cite book|title=''Simulation – The practice of model development and use''|author=Stewart Robinson|publisher=Wiley|year=2004}}[https://spinthewheel.io]</ref> Between consecutive events, no change in the system is assumed to occur; thus the simulation time can directly jump to the occurrence time of the next event, which is called '''next-event time progression'''.

In addition to next-event time progression, there is also an alternative approach, called '''incremental time progression''', where time is broken up into small time slices and the system state is updated according to the set of events/activities happening in the time slice.<ref name="matloff">{{cite web|last=Matloff|first=Norm|title=Introduction to Discrete-Event Simulation and the SimPy Language|url=http://heather.cs.ucdavis.edu/~matloff/156/PLN/DESimIntro.pdf|access-date=24 January 2013}}</ref> Because not every time slice has to be simulated, a next-event time simulation can typically run faster than a corresponding incremental time simulation.

Both forms of DES contrast with [[continuous simulation]] in which the system state is changed continuously over time on the basis of a set of [[Differential equation|differential equations]] defining the rates of change offor state variables.

In the past, these three types of simulation have also been referred to, respectively, as: event scheduling simulation, activity scanning simulation, and process interaction simulation. It can also be noted that there are similarities between the implementation of the event queue in event scheduling, and the [[Scheduling (computing)|scheduling queue]] used in operating systems.

== Example ==

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===Events list===

{{redirect|Future event list|lists of future events|Timelines of the future}}

The simulation maintains at least one list of simulation events. This is sometimes called the ''pending event set'' because it lists events that are pending as a result of previously simulated event but have yet to be simulated themselves. An event is described by the time at which it occurs and a type, indicating the code that will be used to simulate that event. It is common for the event code to be parametrized, in which case, the event description also contains parameters to the event code.{{cn|date=March 2022}} The event list is also referred to as the ''future event list'' (FEL) or ''future event set'' (FES).<ref>{{Cite journal |last1=Park |first1=Hyungwook |last2=Fishwick |first2=Paul A. |date=2010|title=A GPU-Based Application Framework Supporting Fast Discrete-Event Simulation |url=http://journals.sagepub.com/doi/10.1177/0037549709340781 |journal=Simulation |language=en |volume=86 |issue=10 |pages=613–628 |doi=10.1177/0037549709340781 |s2cid=9731021 |issn=0037-5497|url-access=subscription }}</ref><ref>{{Cite web |last=Dannenberg |first=Roger |title=An Introduction to Discrete-Event Simulation |url=https://www.cs.cmu.edu/~music/cmsip/readings/intro-discrete-event-sim.html |access-date=2022-03-11 |website=[[Carnegie Mellon School of Computer Science]]}}</ref><ref>{{Cite web |last=Güneş |first=Mesut |title=Chapter 3: General Principles |url=https://www.mi.fu-berlin.de/inf/groups/ag-tech/teaching/2012_SS/L_19540_Modeling_and_Performance_Analysis_with_Simulation/03.pdf |access-date=2022-03-11 |website=[[Freie Universität Berlin]]}}</ref><ref>{{Cite journal |last1=Damerdji |first1=Halim |last2=Glynn |first2=Peter W. |date=1998 |title=Limit Theory for Performance Modeling of Future Event Set Algorithms |url=https://www.jstor.org/stable/2634704 |journal=Management Science |volume=44 |issue=12 |pages=1709–1722 |doi=10.1287/mnsc.44.12.1709 |jstor=2634704 |issn=0025-1909|url-access=subscription }}</ref>

When events are instantaneous, activities that extend over time are modeled as sequences of events. Some simulation frameworks allow the time of an event to be specified as an interval, giving the start time and the end time of each event.{{cn|date=March 2022}}

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* [[Stochastic process]] and a special case, [[Markov process]]

* [[Queueing theory]] and in particular [[birth–death process]]

* [[DEVS|Discrete Event System Specification]]

* [[Transaction-level modeling]] (TLM)

Computational techniques: