Now let's shift to a CPU subsystem with just a couple of cores. Figures 4-5 and 4-6 are models based on a 2-CPU core system. If you graphically compare the 12-CPU core system with the 2-CPU core system, you will immediately notice a difference. The 12-CPU core system (Figure 4-4) response-time curve has a much more distinct elbow; that is, the system's response-time curve is very flat until it enters the elbow of the curve, and then response time skyrockets. In stark contrast, the 2-CPU core system graph (Figure 4-6) response-time curve elbow is more gradual. In fact, if you look at the queue time value in Figure 4-5, you'll notice that queue time exists even during a light workload. The situation is even more apparent with a single-CPU core system. This is why experienced DBAs know they can run a system with more cores at a higher CPU busyness than a system with just few cores!

Figure 4-5. Numerically, on a two-CPU core system, queuing accounts for 10% of response time at around 32% busy. Just as with Figure 4-3, when the system busyness exceeds 100%, queue time becomes infinite.

Figure 4-6. Graphically, on a two-CPU core system, queuing doesn't look like an issue until around 65% busy. It's more difficult to make a risk assessment because the response-time curve does not have such a dramatic elbow as systems with more CPU cores.