Page 173 - ExtendSim User Guide

P. 173

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Output variables
Output Variable QEQ item rank
QEQ halt calculation cycle QEQ next item
AR allocate requirement
Queueing 147
Sorting items using the Queue Equation block
Uses
Defines each item’s rank (position) in the queue. At least one
of the output variables must be of this type.
If set to True, this variable will prematurely halt the current calculation cycle.
Controls which item in the calculation cycle is evaluated next.
If set to True, the current item’s advanced resource (AR) requirement will be allocated to it. See note above.
As mentioned earlier, while you can specify other equation results, one of the output variables has to be of the type QEQ item rank. Furthermore:
• Only the results for the item ranked best will be output.
• You can use more than one QEQ item rank variable; the secondary ranking variable will be used to arbitrate in the case of tied ranking.
☞ These variables are only available for the item that is currently being released.
Ranking rules
The Release popup on the Options tab provides the following selections to determine which item should be released first:
• Highestrankeditem.Eachitem’srankvalueiscalculatedbytheequationandtheitemwith the highest ranking is allowed to exit.
• Lowest ranked item. Each item’s rank value is calculated by the equation and the item with the lowest ranking is allowed to exit.
• ThefirstTruerankeditem.Theequationwillbecalculatedonceforeachitemuntileitheran item receives a True ranking or there are no items left to evaluate.
• AllTruerankeditems.AllitemsreceivingaTruerankingaremovedtoaninternaldepar-
tures list where they are immediately and permanently available to be moved downstream.
• Theresultsaresavedforeachiteminthedepartureslist.Theseresultsareused(or broadcast) at the time the item exits the queue.
• Itemsinthedepartureslistarenolongerevaluatedbytheequation.Thiscanreducethe number of times an item is evaluated, reducing run times.
• Items are released in FIFO order.
Least dynamic slack
Least dynamic slack is a ranking rule for queues that tends to reduce the “lateness” of a sequence of orders. Slack is defined as the due date minus the remaining processing time. In essence, it is the “float” that is available before the item is due to be completed. When using slack to sort items, priority is given to those items that are closest to being late. In a model that represents orders for services or goods, choosing the order with the least dynamic slack tends to minimize the number of late orders.
Discrete Event
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