* 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.

* Idle time or waiting for IO: When the CPU cores are not busy doing work, they could be truly just sitting around doing nothing, or idle. Or, as with many Oracle systems, they are waiting for data from the IO subsystem. When CPUs are waiting for IO, the time classification is commonly called wait for IO, or wio or wa, for short. When idle time drops to around 30%, transactions start queuing for CPU resources, any response time related to the CPU subsystem starts increasing, and end user performance begins to degrade.

If you ever see system time surpass user time, either contact your system administrator or casually give your pager to your colleague and walk away. When this occurs, which is very unusual, the operating system is spending more time managing itself than doing work for you. The operating system can actually continue spending more and more time self-managing until it finally shuts down.