Modern operating systems are multi-user and multitasking, which means that multiple users and multiple tasks can be using the computer at any given time. Typically you'll have one person using a desktop system running any number of applications or many users using many applications on a server.
The amount of time devoted to tasks largely depends on how intensive the task is. Some tasks require higher priority than others; for instance, if you were compiling a large software package you didn't need immediately, that priority should probably be lower than your Web browser or e-mail client.
Each process has a niceness value associated with it, which is what the kernel uses to determine which processes require more processor time than others. The higher the nice value, the lower the priority of the process. In other words, the "nicer" the program, the less CPU it will try to take from other processes; programs that are less nice tend to demand more CPU time than other programs that are nicer.
The priority is noted by a range of -20 (the highest) to 20 (the lowest). Using ps, you can see the current nice value of all programs:
$ ps axl
F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND
4 0 1 0 16 0 2648 112 - S ? 0:01 init 
1 0 2 1 34 19 0 0 ksofti SN ? 0:02 [ksoftirqd/0]
5 0 3 1 10 -5 0 0 worker S< ? 0:00 [events/0]
You can see that init has a nice value of 0, while other kernel tasks associated with PID 2 and 3 have a nice value of 19 and -5 respectively.
Typically, a program inherits its nice value from its parent; this prevents low priority processes from spawning high priority children. Having said that, you can use the nice command (as root or via sudo) with the command you wish to execute in order to alter its nice value. Here is a short illustration:
# ps axl | grep axl | grep -v grep
4 0 30819 30623 15 0 4660 772 - R+ pts/0 0:00 ps axl
# nice -10 ps axl | grep axl | grep -v grep
4 0 30822 30623 30 10 4660 772 - RN+ pts/0 0:00 ps axl
You can see there that the nice value, represented by column six, has been altered. You can also use the renice command to alter running processes. In the following example, vim was started to edit the file foo and began with a default nice value of 0. Using renice, we can change its priority:
# ps axl | grep vim | grep -v grep
0 0 30832 30623 16 0 15840 3140 - S+ pts/0 0:00 vim foo
# renice -5 30832
30832: old priority 0, new priority -5
# ps axl | grep vim | grep -v grep
0 0 30832 30623 15 -5 15840 3140 - S<+ pts/0 0:00 vim foo
Here, we have adjusted the priority of vim, giving it a slightly higher priority. Renice operates on the process ID, so using grep, we determined that vim is process ID 30832 and saw that the nice value was 0. After executing renice, the nice value is now -5.
Standard caveats apply: Only root can alter the nice priority of programs. So if you find that your compilation is taking too much CPU from other activities, consider renicing the parent process via root. Subsequent children should have a better nice value, or you can even start the compilation (or any other activity) with nice, specifying an appropriate nice value. You can also use renice to renice all programs belonging to a process group or user name/ID.
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Vincent Danen works on the Red Hat Security Response Team and lives in Canada. He has been writing about and developing on Linux for over 10 years and is a veteran Mac user.