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198 Processing
Multitasking
Explicit Shutdown model
The Explicit Shutdown model represents a buffer downstream from a machine that reaches a limit. So that the queue length will control shutdown events, the F connector from the Queue block is connected to the SD input connector on the Activity block.
Explicit Shutdown model
For this model the Queue’s limit is specified in its dialog; whenever the Queue is full, the F connector outputs 1. The Activity’s Shutdown tab indicates that the SD (shutdown) is: value input connection and that therefore the Activity stays down until SD value < 0.5. This causes the Activity to stay shut down for as long as the Queue is full.
This is an example of how to have downstream factors affect upstream activities. If you exam- ine the model closely, you see that the last machine is processing so slowly that Queue 3 quickly reaches its limit of 5 items. Since a Queue cannot take in any more items while it is full, the middle Activity is blocked (cannot process a new item until an item is removed from Queue 3). However, the first Activity continues to process items, filling Queue 2. By explicitly shutting down the first Activity, you affect where items are stockpiled and which Activities are shut down when one of them is blocked.
☞ Please also see “The Shift block” on page 232 for examples of activity and resource allocation that are tied together and scheduled as “Shifts.”
Multitasking
The Activity block has a checkbox in its Process tab
that allows the block to simulate multitasking. Choos-
ing this option means that the block’s available time
to process items (its Delay time) must be divided
between each of the items in the block. This causes
each item to take longer to finish processing and
leave. Examples of multitasking include computer
processors or a person who is working on multiple tasks at the same time.
With multitasking, if only one item is in the block the actual processing time will be exactly the same as the original delay time specified by the block. If two items are in the block, each item will take twice as long as the original specified processing time. If three items are in the block, their processing times will be multiplied by three, and so forth. This is equivalent to situations where a single server or operator has to divide their available time between multiple customers or tasks.
The changes to the processing time occur dynamically as items enter and leave the block. When new items enter the Activity, the remaining delay times for all of the items in the block will become progressively longer. As processed items leave the block, the delays for the remaining items will become shorter.
Choosing multitasking in an Activity
Discrete Event


































































































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