Of the four subsystems we will dig into, the CPU is probably the easiest to understand. Yet, identifying and understanding CPU contention breeds insights into every part of your work. When you look at the CPU subsystem with a response-time analysis perspective, everything seems to fall very neatly into place. Service time, queue time, and the run queue all make sense, and you can see how this translates into the Oracle system. And you'll also find that the basic concepts of queuing theory are blatantly exhibited. It's like a living science experiment!

Models are a wonderful way to distill something very complex into something we can quickly understand. The CPU subsystem model shown in Figure 4-1 provides surprising insights.

In Figure 4-1, the CPU subsystem contains four CPU cores.1 We model a CPU subsystem with a single queue. From our abstraction view, every transaction enters into the single CPU queue and waits for any available CPU core to be serviced. Figure 4-1 shows only one transaction is being serviced, and no transactions are waiting in the queue. This means three of the four cores are not servicing transactions and are therefore idle.