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.

* Only one of the four CPU cores is busy servicing transactions. Said another way, 25% of the CPU cores are busy.