Processes compete for execution time. Scheduling, one of the key elements in a time-sharing system, determines which of the processes executes next. Although hundreds of processes might be present on the system, only one actually uses a given CPU at any given time. Time sharing on a CPU involves suspending a process and then restarting it later. Because the suspension and resumption of active processes occurs many times each second, it appears to the user that the system is performing many tasks simultaneously.
Unix attempts to manage the priorities of processes by giving a higher priority to those that have used the least amount of CPU time. In addition, processes that are waiting on an event, such as a keyboard press, get higher priority than processes that are purely CPU-driven.
On any large system with a number of competing user groups, the task of managing resources falls to the system administrator. This task is both technical and political. As a system administrator, you must understand your company goals to manage this task successfully. When you understand the political implications of who should get priority, you are ready to manage the technical details. As root, you can change the priority of any process on the system by using the nice or priocntl commands. Before you do this, you must understand how priorities work.