While the examples used in this section are based on an entire Oracle instance activity, everything described can also be applied to a single session or a group of sessions. For example, instead of gathering CPU consumption and wait time from v$sysstat, v$sys_time_model, and v$system_event, when focusing on a particular session or group of sessions, use v$sesstat, v$ses_time_model, and v$session_event. Obviously, to calculate operating system utilization, the v$osstat view will have to be used. But a session's or group of session's contribution to the utilization can be calculated in the same way as the Oracle instance CPU utilization (which is simply called Oracle CPU utilization).

When creating a response-time graph representing a real system, it is important to use an appropriate unit of work. For your graph to provide value-mimic and show any relation to reality-it must use a unit of work that relates to the queue time issue. For example, as Table 9-2 shows, if the bottleneck is CPU, logical IO processing will mostly likely correlate very well with CPU consumption. If the bottleneck is IO, the number of SQL executions, the number of block changes, or the number of physical block reads may correlate very well with the IO activity.

If you have multiple samples (for example, you are running reports, pulling from the Statspack tables), you will know if a good unit of work has been chosen because the resulting graph will look somewhat like a response-time curve. As Figures 9-9 and 9-10 demonstrate, it won't be perfect, but it should have an elbow in the curve.