We do not gather service time directly, but instead derive it from existing data. It turns out that nearly always (and fortunately for us), we have parameters for all but the service time. As an example, let's use the data contained in Figures 9-2 and 9-6. Figure 9-2 shows that during the Statspack interval, on average, 414.9 user calls were processed each second. This will be our arrival rate: 414.9 uc/s. Based on Figure 9-6, we know the number of CPU cores is two and as calculated in the operating system CPU utilization at 100%. Solving the utilization formula for the service time, plugging in the numbers, and converting time to milliseconds, we have the following calculation:

Notice if we are careful with the units, the service time naturally results in the unit of time in the numerator and the unit of work in the denominator.

Deriving the IO service time based on the utilization formula is fraught with problems because of non-Oracle IO caching. Even more problematic is knowing the actual number of active IO devices dedicated to an Oracle instance. But it gets worse. Having other IO activity on a specific instance's database files IO device further degrades the service time calculation quality.