ACT CPU Time
CPU time is a valid charging resource, used alongside z/OS (paging) and multiprocessor systems (spin locks and so on).
rm
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.