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nice (pronounced /ˈnaɪs/) is a command found on UNIX and other POSIX-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 nice value of −20 is the highest priority and 19 is the lowest priority. The default nice value for processes is inherited by 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 nice value to a smaller (higher priority) value.

There is also a renice command, which is used to change the priority of a process that's 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's priority. Thus a process run with nice +15 will receive 1/4 of the CPU time allocated to a normal-priority process: (20-15)/(20-0)=1/4. On the BSD 4.x scheduler, on the other hand, the ratio in the same example is more like ten to one.

nice
getpriority(2)

 use POSIX ();
 POSIX::nice(7); # like the renice shell command; increase niceness level by 7
 my $prio = getpriority(0,0); # like the C function

• GNU Core Utilities
• top
• ps
• kill

• nice(1): run a program with modified scheduling priority – Linux User's Manual
• renice(8): alter priority of running processes – Linux Programmer's Manual

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