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.

Oracle is limited in its perception of Oracle process CPU consumption. While great strides have been made with the system time model, there are still two areas where Oracle can misrepresent CPU consumption. Keep in mind that these areas are of no consequence when the CPU subsystem is not massively bottlenecked.

The first area of misrepresentation is because of inefficient data collection tools. While this may not be significant, or even measurable, during times of low CPU utilization, when the database server's CPU subsystem activity becomes extremely intense, executing collection-related commands take a relatively long time. If it takes 5 seconds to ask the database, "What time is it?" that 5 seconds can throw off your analysis. So be aware that during extremely high CPU utilization (90% and greater) reporting error could be significant.