Following our example of a raging CPU bottleneck, we will use logical IOs as our unit of work. Logical IOs consist of all buffer touches. Oracle distinguishes current mode buffer touches by the db block gets instance activity statistic, and the consistent mode buffer touches are signified by the consistent gets statistic. These two statistics will be combined to produce a single logical IO statistic. Based on the instance statistics shown in Figure 9-2, 1,307,632,010 logical IOs occurred during the Statspack reporting interval.

As detailed in the previous sections, for each of your samples (perhaps a single sample or hundreds), get the sample interval time, total CPU consumption (total service time), total non-idle wait time (total queue time), and workload for your selected unit of work (total arrivals). Then for each sample, calculate the arrival rate, service time, and queue time.

Continuing with our example, Figure 9-2 shows the sample interval to be 149.97 minutes in which the logical IO value (sum of db block gets and consistent gets) is 1,307,632,010. Here is the arrival rate math: