2. COST ACCOUNTING CONCEPTS FOR IS ORGANIZATIONS › 2.3 Resource Accounting › 2.3.1 CPU Time

2.3.1 CPU Time

The first attempt in the industry to charge for a specific
resource was to charge a unit of work solely for the amount
of CPU time consumed.  While CPU time is a valid charging
resource, it cannot be used alone, because work uses other
significant resources that must be considered.  z/OS (paging)
and multiprocessor systems (spin locks and so on) add more
inequities to CPU time accounting systems.

z/OS distinguishes between two types of CPU time: Task
Control Block (TCB) time and System Request Block (SRB) time.
In theory, SRB time is the amount of CPU time consumed by the
operating system on behalf of a task, while TCB time is the
CPU time consumed by the task itself.  Thus, SRB time is
assumed to be variable, depending on the multiprogramming
mix, while TCB time is assumed to be constant and therefore a
good accounting measure.  Unfortunately, numerous problems of
both inclusion and exclusion, as well as system architecture,
tend to corrupt the TCB time measurement.

With all its complexity, CPU time is still probably the most
commonly used measurement of data processing resource
consumption.