3.6 Objective 6: Modify Process Execution Priorities

Certain tasks on the system require more execution time than others, and thus deserve to be allocated more frequent execution and longer time slices by the kernel. For this reason, each process is given an execution priority. Usually, you don't need to worry about execution priority because the kernel handles it automatically. Each process' priority level is constantly and dynamically raised and lowered by the kernel according to a number of parameters, such as how much system time it has already consumed and its status (perhaps waiting for I/O -- such processes are favored by the kernel). Linux gives you the ability to bias the kernel's priority algorithm, favoring certain processes over others.

The priority of a process can be determined by examining the PRI column in the results produced from issuing either the top or ps -l commands. The values displayed are relative; the higher the priority number, the more CPU time the kernel offers to the process. The kernel does this by managing a queue of processes. Those with high priority are given more time, and those with low priority are given less time. On a heavily loaded system, a process with a very low priority may appear stalled.

3.6.1 nice

One of the parameters used by the kernel to assign process priority is supplied by the user and is called a nice number. The nice command^[20] is used to assign a priority number to the process. It is so named because it normally causes programs to execute with lower priority levels than with their default. Thus, the process is being "nice" to other processes on the system by yielding CPU time. With this scheme, more "niceness" implies a lower priority, and less niceness implies a higher priority.

^[20] Some shells, not including bash, have a built-in nice command.

By default, user processes are created with a nice number of zero. With this setting, nice doesn't sway the kernel's prioritization scheme one way or another. Positive numbers lower the priority, and negative numbers raise the priority. Nice numbers range from -20 to +19. Any user can start a process with a positive nice number, but only the superuser (root ) can lower a process' nice number to raise its priority. For example, if you have a long-running utility program but don't want to impact interactive performance, a positive nice number will lower the job's priority and improve interactive performance.

Syntax

nice [-n adjustment] [command ]
nice [-adjustment] [command ]

Description

The nice command is used to alter another command's nice number at start time. For normal users, adjustment is an integer from 1 to 19. If you're the superuser, the adjustment range is from -20 to 19. If an adjustment number is not specified, the process' nice number defaults to 10. The command consists of any command that you might enter on the command line, including all options, arguments, redirections, and the background character &.

If both adjustment and command are omitted, nice displays the current scheduling priority, which is inherited.

Example 1

The following command starts a program in the background with reduced priority, using the default nice number of 10:

$ nice somecmd -opt1 -opt2 arg1 arg2 &

Example 2

As superuser, you can start programs with elevated priority. These equivalent commands start the vi editor with a higher priority, which may be necessary for administrative purposes if the system is exceptionally slow:

# nice --10 vi /etc/hosts.deny
# nice -n -10 vi /etc/hosts.deny

Note the double dash ( -- 10) in the first form. The first dash indicates that an option follows, while the second dash indicates a negative number.

Be careful when using nice on interactive programs such as editors, word processors, or browsers. Assigning a program a positive nice number will most likely result in sluggish performance. Remember, the higher the positive number, the lower the resulting priority level.^[21] For that reason, you should try not to assign positive nice numbers to foreground jobs on your terminal. If the system gets busy, your terminal could hang awaiting CPU time, which has been sacrificed by nice.

^[21] The actual scheduling priority used by the kernel is dynamic and takes into account many more factors than the nice value.

3.6.1.1 Changing nice numbers on running processes

The nice command only works to change the nice number for new processes at the time that they're started. To modify a running program, use the renice command.

Syntax

renice [+|-]nicenumber [option] targets

Description

Alter the nicenumber to set the scheduling priority of one or more running target processes. By default, renice assumes that the targets are numeric PIDs. One or more options may also be used to interpret targets as processes owned by specific users.

Frequently used options

-u

Interpret targets as usernames, affecting all processes owned by those users.

-p

Interpret targets as PIDs (the default).

Examples

This command will lower the priority of the process with PID 501 by increasing its nice number to the maximum:

$ renice 20 501

The following command can be used to increase the priority of all of user jdean's processes as well as the process with PID 501:

# renice -10 -u jdean -p 501

In this command, -10 indicates a nice value of negative 10, thus giving PID 501 a higher priority on the system. A dash isn't used for the nice value because the dash could be confused for an option, such as -u.

On the Exam Be sure to know the range and meaning of nice numbers and how to change them for new and existing processes. Also note that nice and renice specify their numbers differently. With nice, a leading dash can indicate a nice number (e.g., -10), including a negative one with a second dash (e.g., -- 10). On the other hand, renice does not need the hyphen.

The renice command is handy for managing groups of processes, particularly to affect user processes by username. In addition, if you're observing processes in top, you may change nice numbers for them interactively from within the top program (discussed in Objective 5) by using the single-keystroke r command. You will be prompted for the PID of the process whose nice number you wish to change and for the new nice number (if you are the superuser, you may enter negative values). The new nice number will be displayed by top in the column labeled NI for the process you specify.