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140 Queueing
Blocks of interest
A queue provides a buffer or waiting line to store items awaiting further processing. Queues can have simple behavior, such as holding items in first in. first out (FIFO) order, or more com- plex behavior, such that items are held and released in groups based on their attributes. You can also set an option in the Queue block’s dialog to specify how long an item will wait until it reneges, or prematurely leaves.
This chapter covers:
• Queueingdisciplines:LIFO,FIFO,Priority,Attribute,andUser-Defined
• Queue/server systems
• Blocking,balking,andreneging
• Sorting items using the Queue Equation block
• Least dynamic slack, minimizing setup, and maximizing service levels
• Using the Queue Matching block to match items into groups based on their attributes
• Viewing and initializing queues with the Queue Tools block
• Displayingqueuecontentsthroughanimating
☞ This chapter’s examples are located in the folder Examples/Discrete Event/Queueing.
Blocks of interest
The following blocks are the main focus of this chapter. Each block’s library and category appears in parentheses after its name.
Queue (Item > Queues)
Stores items until there is downstream capacity. As a sorted queue, holds items in FIFO
or LIFO order, or sorted by their priority or attribute value. As a resource pool queue, holds items in FIFO order.
Queue Equation (Item > Queues)
Stores items. Calculates an equation when it receives an item or when it is triggered by a value connection. When there is downstream capacity, releases items based on the results of the equation.
Queue Matching (Item > Queues)
Has a specified number of internal queues for holding items in separate groups.
Releases a group when there is downstream capacity and the group requirements have been met. This block is useful for matching one type of item with another.
Queue Tools (Utilities > Discrete Event Tools)
When connected to the L (length) output of a queue, views and initializes the queue’s contents. Displays information about item properties in a table. Can add an initial num- ber of items, with specified properties, to a queue.
☞ In this chapter the focus is on using a Queue block to represent a sorted queue. For information about using the Queue block as a resource pool queue see “Resource pool blocks” on page 222.
Queueing disciplines
ExtendSim supports several scheduling algorithms, also known as queueing disciplines, through the queue blocks.
Discrete Event


































































































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