Event list management - a tutorial

  • Authors:

  • James O. Henriksen

  • Affiliations:

  • Wolverine Software Corporation, 7630 Little River Turnpike - Suite 208, Annandale, VA

  • Venue:
  • WSC '83 Proceedings of the 15th conference on Winter Simulation - Volume 2
  • Year:
  • 1983

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Abstract

Proper management of the event list is critically important for achieving execution efficiency in discrete event modelling of computer, telecommunication, and other frequently occurring classes of systems. For such systems, use of a poor event list algorithm can result in horrendous inefficiency. Conversely, replacement of a poor algorithm with a good algorithm can reduce execution times tremendously. For example, in models of telecommunication systems, total run times can easily differ by as much as 5:1, depending on choice of event list algorithm. Many papers have been written about event list algorithms. Unfortunately, the variance in quality of what has been written closely parallels the variance in performance of the algorithms themselves: everything from scholarly works to utter nonsense has been published. Among the persistent myths of simulation is the “fact” that major simulation languages fail to make use of advanced event list algorithms. This is patently false. GPSS/H (Henriksen & Crain 1983) has used an improved algorithm (Henriksen 1977) since 1977, and SLAM (Pritsker 1979) has used an improved algorithm (a variant of the GPSS/H algorithm) since the advent of SLAM II in 1981. Before proceeding, we briefly summarize the sections which follow. Section 1 defines what is meant by the phrase “event list algorithm.” Section 2 presents a hypothetical example, to illustrate the spectacular failure of a naive event list algorithm. Section 3 describes a simple alternative algorithm which is effective when applied to the example of Section 2, but lacks generality. Section 4 presents criteria for evaluating event list algorithms. Section 5 describes some alternative general-purpose algorithms which have been devised, and reviews a number of written works about event list algorithms. Section 6 presents a detailed description of the algorithm used in GPSS/H (Henriksen 1977). Finally, Section 7, offers conclusions.