* Queue time is based on the v$latch view's time_waited column (but v$system_event could have been used, as the values in these two views are consistently within a tenth of a second).

Figure 3-9. Latching acquisition performance data gathered from a real Oracle system, not based on a model. Notice that Oracle's default latching spin and sleep algorithm actually decreases latch acquisition response time per user call until the CPU subsystem becomes saturated. The test was run on a single four-CPU core Linux Oracle 10g Release 2 system.

Figure 3-9 provides deep insights into the efficiency of Oracle's latch acquisition algorithm. Initially, there is some overhead, resulting in a relatively high initial response time. This does not represent a problem, because there is plenty of CPU power available. As concurrency goes up and CPU cycles become increasingly less available, fortunately, economies of scale take effect, resulting in a continual decrease in acquisition service time and response time. But as CPU resources start to become scarce, at 47% CPU utilization, Oracle processes begin sleeping. However, even with the workload increasing, sleep time per user call remains about the same, and CPU time per user calls continues to decrease until around 95% utilization. At this point, CPU resources are in such short supply that queue time becomes increasingly more significant as processes spend more time sleeping.