Long before there was absolute priority (See Absolute Priority), Unix systems were scheduling the CPU using this system. When Posix came in like the Romans and imposed absolute priorities to accommodate the needs of realtime processing, it left the indigenous Absolute Priority Zero processes to govern themselves by their own familiar scheduling policy.
Indeed, absolute priorities higher than zero are not available on many systems today and are not typically used when they are, being intended mainly for computers that do realtime processing. So this section describes the only scheduling many programmers need to be concerned about.
But just to be clear about the scope of this scheduling: Any time a process with an absolute priority of 0 and a process with an absolute priority higher than 0 are ready to run at the same time, the one with absolute priority 0 does not run. If it's already running when the higher priority ready-to-run process comes into existence, it stops immediately.
In addition to its absolute priority of zero, every process has another priority, which we will refer to as "dynamic priority" because it changes over time. The dynamic priority is meaningless for processes with an absolute priority higher than zero.
The dynamic priority sometimes determines who gets the next turn on the CPU. Sometimes it determines how long turns last. Sometimes it determines whether a process can kick another off the CPU.
In Linux, the value is a combination of these things, but mostly it is just determines the length of the time slice. The higher a process' dynamic priority, the longer a shot it gets on the CPU when it gets one. If it doesn't use up its time slice before giving up the CPU to do something like wait for I/O, it is favored for getting the CPU back when it's ready for it, to finish out its time slice. Other than that, selection of processes for new time slices is basically round robin. But the scheduler does throw a bone to the low priority processes: A process' dynamic priority rises every time it is snubbed in the scheduling process. In Linux, even the fat kid gets to play.
The fluctuation of a process' dynamic priority is regulated by another value: The “nice” value. The nice value is an integer, usually in the range -20 to 20, and represents an upper limit on a process' dynamic priority. The higher the nice number, the lower that limit.
On a typical Linux system, for example, a process with a nice value of 20 can get only 10 milliseconds on the CPU at a time, whereas a process with a nice value of -20 can achieve a high enough priority to get 400 milliseconds.
The idea of the nice value is deferential courtesy. In the beginning, in the Unix garden of Eden, all processes shared equally in the bounty of the computer system. But not all processes really need the same share of CPU time, so the nice value gave a courteous process the ability to refuse its equal share of CPU time that others might prosper. Hence, the higher a process' nice value, the nicer the process is. (Then a snake came along and offered some process a negative nice value and the system became the crass resource allocation system we know today).
Dynamic priorities tend upward and downward with an objective of smoothing out allocation of CPU time and giving quick response time to infrequent requests. But they never exceed their nice limits, so on a heavily loaded CPU, the nice value effectively determines how fast a process runs.
In keeping with the socialistic heritage of Unix process priority, a process begins life with the same nice value as its parent process and can raise it at will. A process can also raise the nice value of any other process owned by the same user (or effective user). But only a privileged process can lower its nice value. A privileged process can also raise or lower another process' nice value.
GNU C Library functions for getting and setting nice values are described in See Traditional Scheduling Functions.