Situation C in Figure 4-2 is truly unfortunate. Because there is only one queue, there should never be a situation where processes are waiting in the queue while there are idle CPU cores. Can you imagine standing in a fast food restaurant line, and even though two servers are available, they look at you and tell you to wait? Absurd indeed, and if you ever see this occur (on a computer system), contact your operating system vendor!

* User time: This is when a core is spending time processing code, such as SQL*Plus, the oracle executable, or Perl scripts. Normally, Oracle database CPU subsystems spend about 60% to 90% of their active time in what is called user mode.

* System time: This is when a core is spending time processing operating system kernel code. Virtual memory management, process scheduling, power management, or essentially any activity not directly related to a user task is classified as system time. From an Oracle-centric perspective, system time is pure overhead. It's like paying taxes. It must be done, and there are good reasons (usually) for doing it, but it's not under the control of the business-it's for the government. Normally, Oracle database CPU subsystems spend about 5% to 40% of their active time in what is called system mode. If you're from a non-Unix background, you may be more familiar with the term kernel mode or privileged mode, which is, in essence, system time.