Page 333 - ExtendSim User Guide
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In discrete event modeling problems, the number of items that need to be processed through the simulation may be quite large. This will slow down the execution of the model and increase the amount of memory required. It is often possible (and non-destructive to the validity of your results) to scale down the number of items passing through the model. For example, if there is one item representing a single log in a simulation of a lumber mill, the same model could quite possibly run faster, and equally well, with one item representing ten logs.
When you make scaling changes to a model of this nature, it is very important to reflect the changes everywhere in the model. Thus, if an activity that represented a saw in the lumber mill was set to take one time unit to process an item (one log) before, it must now take ten time units to process the same item (ten logs) after the scaling.
☞ While scaling can sometimes be a useful approach, the Rate library is specifically designed to model high volume and/or high speed systems. In most cases, using the Rate library is superior to item scaling. The Rate library is available with the ExtendSim AT and Suite products.
You sometimes want to have all the items available at the beginning of a simulation instead of generating them as the simulation proceeds. For instance, if you need some random orders pre- sented to the model in sorted order, you might want to sort them before the simulation starts. This is difficult under normal circumstances since the first order would begin traveling through the simulation as the second one was being created. There is an easy method that will cause ExtendSim to create lots of items, store them in a queue, and release them.
Tips and Techniques 307
Moving items through the simulation
Using scaling for large numbers of items
Set the initial value in a Resource Item block to the num- ber of items you want to generate. Connect the Resource Item block to a Set block where you attach item properties (priority, attribute, etc.). Then connect to a Queue that sorts based on the desired item property.
When the model is run, all the items will travel from the Resource Item block to the Queue on step zero. This makes the items, with all their properties, available to the rest of the model at the start of the simulation.
☞ If there are many items in the Resource Item block, the status bar may show the phrase Initial- izing Data. As soon as the preprocessing is done, the timer will settle into a more useful num- ber.
Restricting items in a system
As part of a model you may want to have a section composed of a group of blocks in which only one item (or a limited number of items) can be anywhere in the section at a time. For example, assume you are modeling a manufacturing process with a paint room. There are many blocks that represent the steps in the paint room but only two items are allowed in the entire paint room at a time. New items must be restricted from entering the room until one or more items leave.
When set to Type: area gating, the Gate block is perfect for this because its sensor connector tells it each time an item has reached the end of a system. The number of items allowed are set in the Gate block’s dialog; in this case, two. Put the Gate block at the beginning of a system; at the end of the system, run a parallel connection from the output of the last block to the Gate block’s sensor connector.
Discrete Event

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