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Resources and Shifts
Modeling resources
resource management system is an advanced tool that is only available in the ExtendSim AT and ExtendSim Suite packages. See “Advanced Resource Management” on page 238.
These methods are compared in the table on page 269.
Implicit methods
In addition to the explicit methods listed above, ExtendSim provides additional ways to model resources, as discussed in the “Other methods for modeling resources” section of the User Guide.
☞ Resource-type blocks should only be used in a model if their presence is required for the sys- tem. Otherwise, use an implicit method.
Resource Pool method
The resource pool blocks in the Item library (Resource Pool, Queue, and Resource Pool Release) cause restraints to be placed on the flow of items in the model based on the availabil- ity or lack of resources. The Resource Pool block maintains a count of the number of resources that are currently available for use. When an item enters a Queue block that is in resource pool queue mode, the Queue will query the resource pools to determine if the required number of specified resources are available. If so, the number of resources currently available will be dec- remented in the appropriate Resource Pool block, and the item that requires the resource will be released from the Queue. If the required number of resources are not available, the item will wait in the Queue until resources become available.
In a closed system, the resources are returned to the Resource Pool block by passing the “resourced” item through a Resource Pool Release block. In an open system, such as for a con- sumed resource, the resource is not returned to the pool but is removed from the system when the item exits. Closed and open systems are discussed on page 230.
☞ Since resources are not returned to the originating block in an open system, statistical calcula- tions such as utilization cannot be accurately determined.
Advantages and disadvantages of using resource pools
Advantages
• TheResourcePoolblockdoesnotrequireanyconnectionstootherblocksinamodel. Because of this, using resource pools to model resources (as opposed to using the resource item/batching method that will be described later) is more flexible when the same resource can be used in many different places or when an item can use any one of a group of resources.
• The resource pool method does not require complex routing of resource items because the resources are not actual items but merely constraints on the flow of items through the model.
• When items wait for resource pool units, they can be ranked by priority or FIFO order. The Resource Pool block is able to globally allocate the resource pool unit to the highest ranked item.
Disadvantages
• Theresourcepoolmethoddoesnotallowtheuseofattributestotrackinformationaboutthe individual resources. To use attributes, you must use resource items; this is shown in “Resource Item method” on page 227.
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