Nice (Unix)

nice is a program found on Unix and Unix-like operating systems such as Linux. It directly maps to a kernel call of the same name. nice is used to invoke a utility or shell script with a particular priority, thus giving the process more or less CPU time than other processes. A niceness of −20 is the highest priority and 19 is the lowest priority. The default niceness for processes is inherited from its parent process and is usually 0.

nice becomes useful when several processes are demanding more resources than the CPU can provide. In this state, a higher-priority process will get a larger chunk of the CPU time than a lower-priority process. Only the superuser (root) may set the niceness to a smaller (higher priority) value. On Linux it is possible to change /etc/security/limits.conf to allow other users or groups to set low nice values.

If a user wanted to compress a large file, but not slow down other processes, they might run the following:

The exact mathematical effect of setting a particular niceness value for a process depends on the details of how the scheduler is designed on that implementation of Unix. A particular operating system's scheduler will also have various heuristics built into it (e.g. to favor processes that are mostly I/O-bound over processes that are CPU-bound). As a simple example, when two otherwise identical CPU-bound processes are running simultaneously on a single-CPU Linux system, each one's share of the CPU time will be proportional to 20 − p, where p is the process' priority. Thus a process run with nice +15 will receive 25% of the CPU time allocated to a normal-priority process: (20 − 15)/(20 − 0) = 0.25. On the BSD 4.x scheduler, on the other hand, the ratio in the same example is about ten to one.

The related renice program can be used to change the priority of a process that is already running.

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