In the section on CPU contention in Chapter 4, I introduced how to gather information about the operating system CPU consumption, utilization, and the CPU run queue. Here, we will focus on Oracle's CPU consumption. Obviously, Oracle's CPU consumption is a subset of all CPU consumed on the database server. For example, if the operating system CPU utilization is 65% with only a single Oracle instance on the box, it's a good guess that Oracle is consuming around 55% of all the CPU available on the database server. But in this section, we go a step further.

To complete an ORTA, we need a better understanding of where Oracle consumes CPU. For example, are the Oracle background processes consuming an unusual amount of CPU, or perhaps parsing is the culprit. Just as with wait event classification, Oracle allows a couple CPU-that is, service time-classifications. How we gather and classify Oracle CPU time and how that relates to the database server is what this section is about.

There are three approaches to gathering data about Oracle CPU consumption. The first is the traditional approach based on v$sysstat, the second is based on v$sys_time_model, and the final approach (which is covered in the final section in this chapter) is based on v$active_session_history. But before looking at the approaches, you should understand how Oracle perceives Oracle process CPU consumption.