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nice (pronounced /ˈnaɪs/) is a program found on Unix and Unix-like operating systems such as Linux. nice directly maps to a kernel call of the same name. For a given process, it changes the priority in the kernel's scheduler. 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, 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. If the CPU can deliver more resources than the processes are requesting, then even the lowest priority process can get up to 99% of the CPU. 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.[1]

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

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 \tfrac{1}{4} of the CPU time allocated to a normal-priority process: \tfrac{20-15}{20-0}=\tfrac{1}{4}. On the BSD 4.x scheduler, on the other hand, the ratio in the same example is about ten to one.

Linux also has an ionice program, which affects scheduling of I/O rather than CPU time.

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