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Processing 193
Interrupting processing
The first four options are only available when a value connection is made to PE; the last option is only available, and is the only choice, if an item output is connected to PE.
Preempting model
For instance, an Activity and Queue (set to Sort by: priority) can be used in conjunction with a Decision block (Value library) in such a way that lower priority items being processed may be preempted to make room for higher priority items as they arrive at the Queue. Note that it is not guaranteed that a lower priority item will be preempted. If “Preempt only if block is full” is checked, items will be preempted only if the Activity block is full.
In the Preempting model, the Queue reports the priority of the item that is about to leave and the Activity reports its lowest priority item; this information is sent to the Decision block. If it is determined that there is a higher priority item in the Queue than is being processed by the Activity, a True signal (a value greater than 0.5) is sent to the Activity’s PE input. Notice that the Activity’s dialog is set to When signal is received at PE input, preempt... the item with the lowest priority and to Preempt only if block is full.
Preempting model
Unless they are preempted, items arriving to the Activity block are processed for the time indi- cated in the block’s dialog. In block’s Preempt tab, Store remaining time in attribute: remainingTime is selected. If an item is preempted, the Activity attaches the remaining pro- cessing time to the item as an attribute named remainingTime. Since the Activity also has Use this attribute as delay checked, when the preempted item returns to the Activity block it will process only for the time indicated by the remainingTime attribute.
Shutting down
Employee breaks, equipment maintenance, inventory-taking closings, and tool failures all involve interruptions in activities for a period of time called downtime. If interruptions are sig- nificant, models should include provisions for shutting down activities to avoid overly optimis- tic predictions.
Shutdowns involve a temporary or permanent halting to the processing of items currently in the Activity block. The block’s Shutdown tab has settings to determine which items should have their processing shut down, how long to interrupt the processing, and what to do with the items in an Activity when the shutdown occurs.
You can shut down activities at a scheduled time, such as for vacations or machine mainte- nance, or it can be a random occurrence, such as for equipment failures or emergency leaves. Activities can also be shut down based on some factor in the model, for instance when a down- stream Queue is full. Like shutdown occurrences, the duration of the downtime can be a con-
Discrete Event

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