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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. == Use and effect == 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.〔(limits.conf man page )〕 If a user wanted to compress a large file, but not slow down other processes, they might run the following: $ nice -n 19 tar cvzf archive.tgz largefile 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nice (Unix)」の詳細全文を読む スポンサード リンク
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