On a 12-CPU core system, the elbow of the curve occurs at around 85% busy. With fewer cores, queuing becomes significant much sooner. And with more cores, CPUs can be much busier before queuing becomes significant.

Look closely near the elbow of the curve in Figure 4-4. Notice that once we enter the elbow of the curve, response time skyrockets. I call this "the wall." Any senior DBA will have experienced this.

The situation begins with the system performing wonderfully. It's running so well that the DBA is asked to allow another batch job or two to run concurrently. What the DBA doesn't realize is that the system is operating just before the elbow of the curve. So the DBA decides to allow the increased workload to enter the system-and bam!-performance takes a significant drop. On a 12-CPU core system, it takes only a relatively small increase in the workload to send your system deep into the elbow of the curve. If the system were not operating near the elbow of the curve, then the situation would have been very different. This is perfect example of how understanding just a little queuing theory can make a big difference. If the DBA had known the number of cores, the CPU busyness, a little queuing theory, and some workload management control, he would have never allowed the increased workload. (I will delve deeper into practically applying queuing theory in the last chapter.)