Graphics Programs Reference
In-Depth Information
7
In this chapter we show how GSPN models can account for activities with
generally distributed duration. An expansion of timed transitions by means
of subnets that approximate the desired delay distributions is proposed with
a simple example. The aim of this chapter is that of showing the possibility
of accounting for general distributions without the need of special solution al-
gorithms, while highlighting the subtleties that must be considered to avoid
the construction of incorrect models. The technique allows a correct mod-
elling of the race firing policies and outlines the importance of representing
interruptions and preemptions of activities.
The example chosen to illustrate this possibility is a simple central server
system in which the CPU can be interrupted by the arrival of higher pri-
ority tasks as well as by failures. The implications that these two types of
phenomena may have on the work carried on by the CPU are discussed and
proper modelling representations are developed to show the problems that
may arise when non-exponential firing time distributions are considered.
7.1
The Central Server Model Example
The GSPN modelling formalism described in the first part of this topic
assumes that timed transitions have negative exponential firing time dis-
tributions. This choice was motivated by the desire of ensuring that the
qualitative properties exhibited by the models when time considerations are
ignored (untimed nets with priorities) remain valid also when time is ex-
plicitly considered in the models and by the simplicity of the underlying
stochastic process. With these characteristics, GSPN models can be used
for both validation and performance evaluation purposes.
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