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.

The other area of misrepresentation has to do with time in the CPU run queue. When Oracle reports that a process has consumed 10 ms of CPU time, Oracle does not know if the process actually consumed 10 ms of CPU time or if the process first waited in the CPU run queue for 5 ms and then received 5 ms of CPU time.1 Oracle perceives only 10 ms of time. If this occurs, you will most likely notice it because the Oracle CPU time consumed will be greater than the CPU subsystem's capacity. For example, if you ran a response time-focused report over a duration of 60 seconds, and if the single-CPU core subsystem were heavily overworked, Oracle could report that Oracle processes consumed 65 seconds of CPU time. Yet, we clearly know the CPU subsystem is limited to supplying up to 60 seconds of CPU time and no more. And let's not forget that the operating system consumes CPU to manage the database server. So, even if Oracle is reported to consume 90% of the available CPU, it's a good guess that the CPUs are on average 100% busy. Again, this is rare, but on an extremely CPU bottlenecked system, when the CPU run queue is significantly larger than the number of CPU cores, it can happen.