Nice value and priority and relations

The priority of a process in linux is dynamic: The longer it runs, the lower its priority will be. A process runs when its actually using the CPU - most processes on a typical Linux box just wait for I/O and thus do not count as running.

The priority is taken into account when there are more processes running than CPU cores available: Highest priority wins. But as the winning process looses its proirity over time, other processes will take over the CPU at some point.

nice and renice will add/remove some "points" from priority. A process which has a higher nicevalue will get lesser CPU time. Root can also set a negative nice value - the process gets more CPU time.

Example: There are two processes (1 and 2) calculating the halting problem and one CPU core in the system. Default is nice 0, so both processes get about half of the CPU time each. Now lets renice process 1 to value 10. Result: Process 2 gets a significant higher amount of cpu time as process 1.

Note: In modern desktops there is plenty CPU time available - they are fast these days. Unfortunately HDDs are still relativeley slow on random I/O, so even a nice process can generate enough I/O traffic to significantly slow down a system.

Processes have an initial priority specified as the nice value. This value is stored in the static_prio member of the process’s task_struct. The value is called the static priority because it does not change from what the user specifies. The scheduler, in turn, bases its decisions on the dynamic priority that is stored in prio. The dynamic priority is calculated as a function of the static priority and the task’s interactivity.

HIGH PRIORITY – - – - – > – - – - – > – - – - – > – - – - – > – - – – LEAST PRIORITY
……..real time priority (static priority)…….| …. nice value (dynamic priority) …..
99 ……………………….. 50 ……………………… 1 | -20 …….. -10 …….. 0 …….. 10 ……. 19