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This chapter is from the book

This chapter is from the book

Configure Kernel Options

The Linux kernel is highly configurable. You can make changes to the kernel by using parameters and kernel modules. This section covers these configuration features.


A kernel parameter is a value that changes the behavior of the kernel.


You can view and change parameters by using the sysctl command. For example, to view all the kernel and kernel module parameters, execute the sysctl command with the -a option:

[root@onecoursesource ~]# sysctl -a | head
kernel.sched_child_runs_first = 0
kernel.sched_min_granularity_ns = 1000000
kernel.sched_latency_ns = 5000000
kernel.sched_wakeup_granularity_ns = 1000000
kernel.sched_tunable_scaling = 1
kernel.sched_features = 3183
kernel.sched_migration_cost = 500000
kernel.sched_nr_migrate = 32
kernel.sched_time_avg = 1000
kernel.sched_shares_window = 10000000

The name of the parameter (kernel.sched_child_runs_first, for example) is a relative pathname that starts from /proc/sys and has a dot (.) character between the directory and filename rather than a slash (/) character. For example, the /proc/sys/dev/cdrom/lock file is named using the dev.cdrom.lock parameter:

[root@onecoursesource ~]# sysctl -a | grep dev.cdrom.lock
dev.cdrom.lock = 1

You can change the value of this parameter by using the sysctl command:

[root@onecoursesource ~]# sysctl dev.cdrom.lock=0
dev.cdrom.lock = 0
[root@onecoursesource ~]# sysctl -a | grep dev.cdrom.lock
dev.cdrom.lock = 0

It is actually safer to use the sysctl command than to modify the file directly because the sysctl command knows which values for the parameter are valid and which ones are not:

[root@onecoursesource ~]# sysctl dev.cdrom.lock="abc"
error: "Invalid argument" setting key "dev.cdrom.lock"

The sysctl command knows which parameter values are valid because it can look at the modinfo output. For example, the value of the lock file must be a Boolean (0 or 1), according to the output of the modinfo command:

[root@onecoursesource ~]# modinfo cdrom | grep lock
parm:           lockdoor:bool

If you modify the file directly or use the sysctl command, the changes are temporary. When the system is rebooted, the values go back to the defaults, unless you make changes in the /etc/sysctl.conf file. The next section provides more details about /etc/sysctl.conf.


The /etc/sysctl.conf file is used to specify which kernel parameters to enable at boot.


[root@OCS ~]# cat /etc/sysctl.conf
# System default settings live in/usr/lib/sysctl.d/00-system.conf.
# To override those settings, enter new settings here, or
# in an /etc/sysctl.d/<name>.conf file.
# For more information, see sysctl.conf(5) and sysctl.d(5).

In this example, the kernel parameter ip_forward is turned on, which means this machine will act as a router between two networks.

There are thousands of possible kernel settings, including dozens of settings that affect networking. The ip_forward setting is one of the most common network settings.

The parameters that optimize the IO (input/output) scheduler are examples of kernel parameters. Several parameters can be set to change the behavior of the scheduler. This section covers the parameters that are important for the Linux+ XK0-005exam.

To see the current scheduler, view the contents of the /sys/block/<device>/queue/scheduler file (where <device> is the actual device name). Here’s an example:

[root@OCS ~]# cat /sys/block/sda/queue/scheduler
[noop] deadline cfq

The value within the square brackets is the default. To change this, use the echo command, as shown here:

[root@OCS ~]# echo "cfq" > /sys/block/sda/queue/scheduler
[root@OCS ~]# cat /sys/block/sda/queue/scheduler
noop deadline [cfq]

Additional scheduler types include the following:

  • arrow.jpg cfq: The Completely Fair Queuing schedule has a separate queue for each process, and the queues are served in a continuous loop.

  • arrow.jpg noop: This schedule follows the FIFO (first in, first out) principle.

  • arrow.jpg deadline: This is the standard scheduler. This scheduler creates two queues: a read queue and a write queue. It also puts a timestamp on each I/O request to ensure that requests are handled in a timely manner.


A module is a small software program that, when loaded, provides more features and capabilities to the kernel. This section describes the management of modules using the lsmod, imsmod, rmmod, insmod, modprobe, and modinfo commands.


The lsmod command displays the kernel modules that are loaded into memory. This command has no options.

In the output of the lsmod command, each line describes one module. There are three columns of information for each line:

  • arrow.jpg The module name.

  • arrow.jpg The size of the module, in bytes.

  • arrow.jpg The “things” that are using the module. A “thing” could be a filesystem, a process, or another module. In the event that another module is using this module, the dependent module name is listed. Otherwise, a numeric value that indicates how many “things” use this module is provided.


[root@OCS ~]# lsmod | head
Module                   Size       Used by
tcp_lp                   12663      0
bnep                     19704      2
bluetooth                372944     5   bnep
rfkill                   26536      3   bluetooth
fuse                     87741      3
xt_CHECKSUM              12549      1
ipt_MASQUERADE           12678      3
nf_nat_masquerade_ipv4   13412       1   ipt_MASQUERADE
tun                       27141 1


The imsmod command is used to add modules to the currently running kernel.


insmod [module_name]

The exact location of the module needs to be specified. For example:

[root@OCS ~]# lsmod | grep fat
[root@OCS ~]# insmod /usr/lib/modules/3.19.8-100.fc20.x86_64/kernel/
fs/ fat.ko
[root@OCS ~]# lsmod | grep fat
fat                   65107 0

There are no options to the insmod command; however, each module might have modules that can be passed into the module using the following syntax:

insmod module options

The insmod command has two disadvantages:

  • arrow.jpg You have to know the exact location of the module.

  • arrow.jpg If the module has any dependencies (that is, if the module needs another module), it will fail to load.


The rmmod command is used to remove modules from the currently running kernel.


rmmod [options] [module_name]


[root@OCS ~]# lsmod | grep fat
fat   65107 0
[root@OCS ~]# rmmmod fat
[root@OCS ~]# lsmod | grep fat

Modules that are currently in use will not be removed by this command by default.

Key options for the rm command include the following:

  • arrow.jpg -f attempts to force removal of modules that are in use (which is very dangerous).

  • arrow.jpg -w waits for a module to be no longer used and then removes it.

  • arrow.jpg -v displays verbose messages.


The insmod command is used to add modules to the currently running kernel.


insmod [module_name]

The exact location of the module needs to be specified. For example:

[root@OCS ~]# lsmod | grep fat
[root@OCS ~]# insmod /usr/lib/modules/3.19.8-100.fc20.x86_64/kernel/
fs/ fat.ko
[root@OCS ~]# lsmod | grep fat
fat                   65107 0

There are no options to the insmod command; however, each module might have modules that can be passed into the module using the following syntax:

insmod module options

The insmod command has two disadvantages:

  • arrow.jpg You have to know the exact location of the module.

  • arrow.jpg If the module has any dependencies (that is, if the module needs another module), it will fail to load.


The modprobe command is used to add and remove modules from the currently running kernel. It also attempts to load module dependencies.


modprobe [options] [module_name]

When used to remove modules (with the -r option), the modprobe command also removes dependency modules unless they are in use by another part of the subsystem (such as the kernel or a process).

Key options for the modprobe command include the following:

  • arrow.jpg -c displays the current modprobe configuration.

  • arrow.jpg -q causes modprobe to run in quiet mode.

  • arrow.jpg -R displays all modules that match an alias to assist you in debugging issues.

  • arrow.jpg -r removes the specified module from memory.

  • arrow.jpg -v displays verbose messages; this is useful for determining how modprobe is performing a task.


The modinfo command is used to provide details about a module.


modinfo [module_name]


[root@OCS ~]# modinfo xen_wdt
filename: /lib/modules/3.19.8-100.fc20.x86_64/kernel/drivers/watchdog/
license: GPL
version: 0.01
description:   Xen WatchDog Timer Driver
author: Jan Beulich <jbeulich@novell.com>
srcversion:   D13298694740A00FF311BD0
intree:     Y
vermagic:    3.19.8-100.fc20.x86_64 SMP mod_unload
signer:       Fedora kernel signing key
sig_key:     06:AF:36:EB:7B:28:A5:AD:E9:0B:02:1E:17:E6:AA:B2:B6:52: 63:AA
sig_hashalgo:   sha256
parm:           timeout:Watchdog timeout in seconds (default=60)(uint)
parm:            nowayout:Watchdog cannot be stopped once started (default=0) (bool)

One of the most important parts of the output of the modinfo command is the parm values, which describe parameters that can be passed to this module to affect its behavior.

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