For CPU subsystems, you can compare the actual CPU utilization (perhaps gathered from v$osstat or vmstat) with the classic utilization formula. If you picked a good unit of work, the difference should be within 15%. If the bottleneck is the IO subsystem, because of caching and batching, utilization comparison may be interesting, but it is unlikely to closely match or provide much value.

Continuing with our example, the actual CPU utilization, based on the v$osstat statistics shown in Figure 9-6, is 100%. To derive the CPU utilization, enter the calculated arrival rate, service time, and number of CPU cores (also shown in Figure 9-6) as follows:

As we hoped, we are within 10%. If we are not within 15%, we can still use our graph for informational purposes, but for numerically quantifying and anticipating our solution's impact (as described later), it will not be reliable.