Nice (Unix)
Encyclopedia
nice is a program found on Unix
and Unix-like
operating system
s 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 or 20 is the lowest priority. The default niceness for processes is inherited from its parent process, usually 0.
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.
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 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.
Linux also has an ionice program, which affects scheduling of I/O rather than CPU time.
The name "niceness" originates from the idea that a process with a higher niceness value is "nicer" to other processes in the system, as it allows the other processes more CPU time.
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
and Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s such as Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
. nice directly maps to a kernel call of the same name. For a given process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
, it changes the priority
Priority queue
A priority queue is an abstract data type in computer programming.It is exactly like a regular queue or stack data structure, but additionally, each element is associated with a "priority"....
in the kernel's scheduler
Scheduling (computing)
In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...
. A niceness of −20 is the highest priority and 19 or 20 is the lowest priority. The default niceness for processes is inherited from its parent process, usually 0.
Use and effect
nice becomes useful when several processes are demanding more resources than the CPUCentral processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
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
Superuser
On many computer operating systems, the superuser is a special user account used for system administration. Depending on the operating system, the actual name of this account might be: root, administrator or supervisor....
(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.
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 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.
Linux also has an ionice program, which affects scheduling of I/O rather than CPU time.
Etymology
The name "nice" comes from the fact that the program's purpose is to modify a process niceness value. The true priority, used to decide how much CPU time to concede to each process, is calculated by the kernel process scheduler from a combination of the different processes niceness values and other data, such as the amount of I/O done by each process.The name "niceness" originates from the idea that a process with a higher niceness value is "nicer" to other processes in the system, as it allows the other processes more CPU time.