Nice (Unix) - Use and Effect

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. 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.

If a user wanted to compress a large file, but not slow down other processes, they might run the following:

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.