5.5: Organizing Entities Waiting for a Resource

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Notice from the discussion in section 5.2 that there is either zero or one event occurrence on the event list corresponding to each entity. If there is no event occurrence, the entity is waiting usually for a resource to become available. Multiple entities may be waiting for the same resource. Thus, it is necessary to maintain lists of waiting entities as well as lists of event occurrences.

Entities wait for a resource that is currently not in the idle state in an ordered list similar to the event list. When a unit of the resource completes its current task or otherwise becomes idle, it will process the first entity in the list. The list is sequenced either by order of entity entry in the list (first-in-first-out or last-in-first-out) or by an entity attribute value (high-value-first or low-value- first).

Suppose entities in the two workstation model have the following attributes:

Time of arrival to the system
Estimated processing time at workstation A

Suppose that at a particular moment in simulation time there are three entities waiting for the workstation A resource. The waiting entities are ordered first-in-first-out as follows.

\[
\begin{array}{l|ll}
\text { Entity } & \text { Time of Arrival } & \begin{array}{l}
\text { Estimated } \\
\text { Processing Time }
\end{array} \\
\hline 101 & 100.0 & 15.0 \\
102 & 110.5 & 9.8 \\
103 & 120.5 & 21.0
\end{array}
\nonumber\]

Alternatively, suppose the entities were sorted by the lowest value of estimated processing time first as follows.

\[
\begin{array}{l|ll}
\text { Entity } & \text { Time of Arrival } & \begin{array}{l}
\text { Estimated } \\
\text { Processing Time }
\end{array} \\
\hline {1 0 2} & 110.5 & 9.8 \\
{1 0 1} & 100.0 & 15.0 \\
{1 0 3} & 120.5 & 21.0
\end{array}
\nonumber\]

Note that the sequence in which entities are processed at workstation A is the same as their order in the queue of workstation A.