Figure 9-8 contrasts an eight-device IO subsystem (solid line) and an eight-CPU core subsystem (dotted line) having the exact same service time. We know their service times are the same because, at a minimal arrival rate when no queuing occurs, their response time is exactly the same. With the understanding that service time does not change, regardless of the arrival rate, we know that any increase in the response time is due to queue time.

Figure 9-8 shows that a CPU subsystem can maintain a fairly consistent response time until it reaches near capacity.2 This means little or no queuing exists until the arrival rate significantly increases. In contrast, IO subsystems start queuing immediately, as reflected in the upward-sloping response-time curve.

Figure 9-8. Shown in the classic response-time curve contrasting an eight-device IO subsystem (solid line) and an eight-CPU core subsystem (dotted line). Even with a perfectly balanced IO subsystem, without advanced algorithms and a significant amount of IO caching, IO requests nearly always contain significant amounts of queue time.