ACT Resource Occupancy Time
The charge for a device should consider the amount of time the device is allocated in addition to its utilization.
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One common approach to account for device usage is to charge for the utilization (I/O interrupts) of a device. This approach, although widely practiced, has some drawbacks. The charge for a device should consider the amount of time the device is allocated in addition to its utilization. For example, a job allocates three tape drives and uses only one of them. With utilization accounting, the job step would incur no charge at all for the two devices that were allocated but never used. As a result, only one part of the true cost is accounted.
Resource occupancy time applies pseudo-elapsed time to the allocated resources. It is in the accounting for dedicated resources that the use of occupancy times is truly valid. Because the user does not have control over the multiprogramming environment, it is not consistent or equitable to charge for a tape drive according to the actual elapsed time of the job step. However, it is valid to charge for that tape drive according to the pseudo-elapsed time or occupancy time, because the user does have control of that measurement. In other words, the dedicated resources can be charged out according to how long the job step would have used them in a single-job environment. This permits each resource to be examined according to its cost and allocation. The following example illustrates charging for dedicated resources.
Job Step A uses five tape drives. The measured times for this job step are: Elapsed Time 60 Mins Occupancy Time 30 Mins CPU Time 11 Mins The charges for this job step are: CPU Time 11 minutes @ $15.00/minute = $165.00 Dedicated Resources: Tape Drives 5 tapes allocated for 30 minutes each = 150 tape occupancy minutes (TOM) @ $0.10/TOM = $ 15.00 _______ Total Step Cost $180.00