Chris Tyler - Fedora Linux

Since processes are usually waiting for data, it's not uncommon for programs to run for only a few seconds a day. I've been using my X display server heavily all day, and it's accumulated less than 30 minutes of CPU time; my POP3 mail server, which is accessed 600 times and transfers several hundred megabytes of data each day, accumulates less than 20 seconds of CPU time a day.

The 2.6 kernels now used in Fedora do fully preemptive scheduling, which means that when data does arrive for a sleeping process, and that sleeping process has a higher priority than the process currently running, the kernel will preempt the running process and immediately schedule the new process for execution (instead of waiting for the currently executing process to reach the end of its timeslice).

The kernel dynamically changes the priority of a process based on the amount of time since it last executed, the amount of time it has executed recently, the amount of I/O it is performing, and the nice value.

To terminate a running process, a numeric signal is sent to that process. To see all of the available signals, use the -l argument to kill , which shows the signal names and numbers:

$ kill -l

1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL

5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE

9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2

13) SIGPIPE 14) SIGALRM 15) SIGTERM 17) SIGCHLD

18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN

22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ

26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO

30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1

36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5

40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9

44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13

48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13

52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9

56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5

60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1

64) SIGRTMAX

Each of these signals has a specific meaning, which can usually be determined from the signal name; for example, SIGHUP is the hangup signal, SIGINT is the interrupt signal (sent when you use Ctrl-C to try to interrupt a program running on a terminal or character-mode VT), SIGFPE is the signal for floating-point exceptions (such as division by zero), and SIGPWR is the signal for a power failure. Most of these signals are generated automatically by the kernel and basic server processes.

In most cases, a process can arrange to catch a particular signal and do something; for example, a text editor might save the current file when SIGHUP is received (the connection to the terminal is lost). If the process has not arranged to catch a signal, a default action is taken. In most cases, the default action is to terminate the process, but the default for some signals (such as SIGPWR ) is simply to ignore the signal and keep on running.

SIGTERM is used to request that a program terminate itself. Most programs catch this signal and clean up before they terminate, deleting any temporary files, saving data if necessary, informing network peers that they are terminating (where appropriate), and so forth. This is the default signal sent by top and kill , and the signal sent by the GNOME System Monitor when you specify End Program.

SIGKILL is uncatchable. It always terminates a program. This is useful when you wish to definitely terminate a program, but it doesn't give the program an opportunity to shut down gracefully, so files and network communications may be left in half-finished states, which may cause future problems. For this reason, it should be used as a last resort. SIGKILL is the signal sent by the Kill Program option in System Monitor and the Kill button in KSysGuard.

Both KDE and GNOME provide panel applets that display a continuous graph of the current CPU load, memory usage, and more. To add this applet to your panel bar, right-click on an empty area on the bar and select "Add to Panel." For GNOME, select System Monitor; for KDE, select Applet→KSysGuard. You can configure the display by right-clicking on it and selecting Preferences or Properties.

The nice command starts a process with a lower-than-normal priority. The priority can be reduced by any value from 1 to 19 using the -n argument; without -n , the priority is reduced by a value of 10. The command to be run is the only other argument required (any additional arguments are used as arguments to that command):

$ nice -n 15 xboard

To raise the priority of a process, you must be root ; supply a negative priority adjustment between 1 (slight boost in priority over normal) to 20 (highest priority):

# nice -n -12 xboard

renice is the tool for this:

$ xboard &

[3] 27365

$ renice 5 27365

27365: old priority 0, new priority 5

$ renice 2 27365

renice: 27365: setpriority: Permission denied

Note that the value used with the nice command is the opposite of what you may usually associate with a priority. Put another way, a nice level of 20 results in a process that isn't very nice to its fellow processes, since it's running at a high priority and hogs the CPU.

Notice that renice does not permit the user to increase the priority of a process, even if the user lowered it in the first place. However, root can set any priority she chooses:

$ renice -5 27365

renice: 27365: setpriority: Permission denied

# renice 2 27365

27365: old priority 5, new priority 2

# renice -5 27365

27365: old priority 2, new priority -5

You can also adjust the priority of processes in System Monitor and KSysGuard using the options on the context menu (right-click on the process you wish to adjust).

When using the shell, you can start a process in the background by placing an ampersand after the command:

$ xboard &

[21771]

$ mc &

[21783]

$

The shell will display the PID of the background process, then immediately present a new prompt, permitting you to enter additional commands before the background command has finished executing.

You can display background processes using the jobs command:

$ jobs

[1]- Running xboard &

[2]+ Stopped . /usr/share/mc/bin/mc-wrapper.sh

Any program that attempts to communicate through the character interface, such as Midnight Commander ( mc ) in this example, will be stopped. Programs that communicate through the graphical user interface, such as xboard , are free to do so while running in the background.