You might have experienced this yourself with your computer systems. Performance is fine, yet the system is really busy. Then for any number of reasons, the system activity increases just a little, and-wham!-performance takes a dive. And you sit back and say, "What just happened? Everything was going fine, and the workload didn't increase that much."

What happened to both of us is that we hit the famous rocket-science-like term, elbow of the curve (also known as the knee of the curve). At the elbow of the curve, a small increase in the arrival rate causes a large increase in the response time. This happens in all queuing systems in some fashion, and it is our job to understand when and under what conditions the elbow of the curve will occur.

As I will detail in Chapter 5, it is very convenient for DBAs when service time is CPU time, and queue time is Oracle wait-interface time. Think of the arrival rate as the Oracle workload. If you are familiar with Statspack or AWR, you know that near the beginning of the report, you can find a series of load profile statistics. Any one of those can represent the arrival rate. When the response-time curve is used for predictive purposes, a model is developed based on one or more of the load profile statistics. Common Oracle arrival rate statistics are user calls, executions, and buffer gets.13