Editing Nice (Unix) (section)

==Use and effect==
<code>nice</code> becomes useful when several processes are demanding more resources than the [[Central processing unit|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 lower value (i.e. a higher priority). On Linux it is possible to change <code>/etc/security/limits.conf</code> to allow other users or groups to set low nice values.<ref>{{man|5|limits.conf|ManKier}}</ref>

If a user wanted to compress a large file without slowing down other processes, they might run the following:
<syntaxhighlight lang="console">
$ nice -n 19 tar cvzf archive.tgz largefile
</syntaxhighlight>

The exact mathematical effect of setting a particular niceness value for a process depends on the details of how the [[Scheduling (computing)|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 <code>nice +15</code>, will receive 25% of the CPU time allocated to a normal-priority process: (20 − 15)/(20 − 0) = 0.25.<ref>{{cite book |title=Operating system concepts |last1=Silberschatz |first1=Abraham |last2=Galvin |first2=Peter B. |last3=Gagne |first3=Greg |date=2013 |publisher=Wiley |location=Hoboken, N.J |isbn=978-1-118-06333-0 |page=294 |edition=ninth}}</ref> On the [[Berkeley Software Distribution|BSD]] 4.x scheduler, on the other hand, the ratio in the same example is about ten to one.{{Citation needed|date=November 2013}}