Page 167 - ExtendSim User Guide
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Queueing 141
Queue/server systems
• FIFO.Whensettobeasortedqueue,theQueueblockcanrepresentafirstin,firstout (FIFO) queue, also known as a first come, first served queue. When set as a resource pool queue, the Queue block represents a FIFO queue for resource pool units. The “MM1 model” on page 142 is an example of a FIFO queue and most of the models in the Discrete Event module use a Queue block in FIFO mode. For more information about resource pools and how the Queue is used as a resource pool queue, see “Resource pool blocks” on page 222.
• LIFO.Whensettobeasortedqueue,theQueueblockcanrepresentalastin,firstout queue. As is true when the Queue is set to FIFO mode, the Queue block automatically takes care of LIFO sorting.
• Priority.Asasortedqueue,theQueueblockcanreadprioritiesandpassitemswiththe highest priority (lowest number) out first. For this to happen, the arriving items must have a priority. Items that have not been assigned a priority in the model have a default priority with a Blank value; they get relegated to the end of the waiting line. To see a Queue sorting items based on priorities, see “Priority queues” on page 142 or “Animating queue contents” on page 153.
• Attribute.Asasortedqueue,theQueueblockcanuseattributevaluestosortitemsinthe queue. In addition, the Queue Matching block allows you to define custom scheduling algo- rithms based on item attributes. It groups items based on certain attributes and releases them as a group once requirements are met. For this sorting rule, items must have attributes assigned to them before entering the Queue. Items that have not been assigned an attribute in the model have a default attribute with a Blank value; they get routed to the end of the wait- ing line. The process for having a Queue sort items based on attributes is similar to the pro- cess for sorting using priorities.
• User-Defined.TheQueueEquationblockallowsauser-definedequationtodecidethesort- ing order for items it holds. This can be used to specify any user-defined criteria for sorting, including Least Dynamic Slack, Minimize Setup, Maximize Service Level, and any other combination of sorting rules. A discussion of these ranking rules and example models start on page 145.
☞ It is important to remember that, except for a FIFO queue, there must be other items in a queue at the same time to allow the queueing disciplines to work appropriately and affect the order of the items. For example, if you set a Queue block to sort by priority, and there is never more than one item in the block at a time, the effect of queueing based on priority is negated.
Queue/server systems
Queue/server systems involve the creation of items which then wait in a queue until they can be processed by one or more servers. The following blocks in the Item library are used to rep- resent queue/server systems:
• TheCreateblockisusedtoprovideitemsatexponentialinterarrivaltimes(andmanyother interarrival times as well).
• AQueueblock,settosortinFIFO,LIFO,orsomeotherorder,holdstheitemsandreleases them in the designated order. It can have a maximum queue length specified in its dialog.
Discrete Event

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