2. COST ACCOUNTING CONCEPTS FOR IS ORGANIZATIONS › 2.3 Resource Accounting › 2.3.5 Resource Occupancy Time

2.3.5 Resource Occupancy Time

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