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152 Queueing
Advanced queue topics
Popup menus at the top of the columns are for selecting which of the item’s prop- erties (attribute, quantity, priority, and so forth) to view. There are also buttons (Up, Down, Destroy) on the View tab that can change the rank of an item in the queue or delete an item from the queue. For instance, in the screenshot to the right an item with a priority of 4 has been moved in front of other items with priori- ties of 1.
Manipulating a queue
To manipulate the items in a queue, run the model, pause the simulation at the desired point, select an item, and use the buttons in the dialog to move or destroy it. (To pause a simulation run, click on any cell in the Queue Tools table or use the Pause button in the ExtendSim toolbar or the Pause menu command.) Leaving this block open (or having a clone on the model work- sheet) while the simulation is running will slow the model down; it is best to close it when not needed.
The View tab of a Queue Tools block is cloned onto the worksheet of the Initializing and View- ing model, discussed in the next topic.
Initializing a queue
Sometimes it is useful to introduce items into the model at the start of the simulation run. The Options tab in a Queue Tools block (Utilities library) can be used to preload a queue with items at start time. Situations where queues might be initialized with items include:
• Reducing start-up bias. By placing items in queues at the start of a simulation, the model begins in a state that is closer to steady-state.
• Importingcurrentsystemstatusinaschedulingmodel.Whenusingsimulationtomodela detailed schedule, it is necessary to start the simulated system with the same work-in-prog- ress as the real system.
To use this block, connect from the L (length) output on a queue to the value input connector on a Queue Tools block. The block’s Options tab has three choices:
• No queue initialization: Items are not added to the queue at the start of the simulation.
• Initialize queue: The queue is initialized with the number of items entered in the number field and property values as specified in the Properties table.
• Initializefromglobalarray:Thenumberofitemsandpropertyvaluesarereadinfroma specified global array. Because global arrays can themselves be initialized from a number of sources including Excel, a database, or the Internet, this is a very useful way to import the contents of a queue from an external source.
Discrete Event

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