Page 287 - ExtendSim User Guide
P. 287

Resources and Shifts 261
Additional information
• Setting a policy that determines the order in which items are evaluated for resource alloca- tion.
• Overriding the resource requirement rank. (The original resource requirement rank is entered on the Requirements tab when the resource requirement is created.)
Resource allocation policy
The top section of the Policies tab is for setting the resource allocation policy. The Resource Manager uses this policy to determine the order in which items will be evaluated for resource allocation.
Typically the resource allocation policy is strictly maintained- a particular resource will not be allocated to a lower priority item if that resource resides in the requirement of a higher priority item. In other words, by default required resources are reserved for higher priority items. How- ever, as discussed below the default policy can be overridden.
Sorting criteria
Based on the resource allocation policy, all the waiting items in the model are sorted into a pri- oritized list. Item properties are used to control the order in which waiting items are sorted in the list. The properties are:
• Item priority – the lowest number, including negative values, is the highest priority
• Value attribute
• DBAddressattribute
• Queuearrivaltime
• Resourcerequirementrank.Whenaresourcerequirementiscreated,itgetsarankingvalue that is set in the Requirements tab. And before a waiting item can be released from an advanced resource queue, it must be associated with a resource requirement. Since each item has a resource requirement, the rank can be used to sort waiting items.
Items can be sorted by any one of these properties, in ascending or descending order. A sec- ondary sorting order is also allowed.
☞ The default is to sort items by the ranking of their resource requirements and then by the time the item arrived to the queue.
Evaluation and allocation
Item evaluation takes place when:
• An item arrives at a queue and is not blocked from leaving
• Aresourceswitchesfromanyotherstatustoidle
• Downstream blocking is alleviated and an upstream item can now leave a queue
The highest priority item is evaluated first; if its full set of required resources is available the resource is allocated to it. If the first item's requirement cannot be met, the next item in the list is evaluated, and so forth. If no item's requirement can be met, allocation is delayed until either more resources become available or until another item with a different requirement submits an order.
Discrete Event

   285   286   287   288   289