Chapter 6. Configuring kernel parameters at runtime

As a system administrator, you can modify many facets of the Red Hat Enterprise Linux kernel’s behavior at runtime. This section describes how to configure kernel parameters at runtime by using the sysctl command and by modifying the configuration files in the /etc/sysctl.d/ and /proc/sys/ directories.

6.1. What are kernel parameters

Kernel parameters are tunable values which you can adjust while the system is running. There is no requirement to reboot or recompile the kernel for changes to take effect.

It is possible to address the kernel parameters through:

  • The sysctl command
  • The virtual file system mounted at the /proc/sys/ directory
  • The configuration files in the /etc/sysctl.d/ directory

Tunables are divided into classes by the kernel subsystem. Red Hat Enterprise Linux has the following tunable classes:

Table 6.1. Table of sysctl classes

Tunable classSubsystem

abi

Execution domains and personalities

crypto

Cryptographic interfaces

debug

Kernel debugging interfaces

dev

Device-specific information

fs

Global and specific file system tunables

kernel

Global kernel tunables

net

Network tunables

sunrpc

Sun Remote Procedure Call (NFS)

user

User Namespace limits

vm

Tuning and management of memory, buffers, and cache

Important

Configuring kernel parameters on a production system requires careful planning. Unplanned changes may render the kernel unstable, requiring a system reboot. Verify that you are using valid options before changing any kernel values.

Additional resources

  • For more information about sysctl, see sysctl(8) manual pages.
  • For more information about /etc/sysctl.d/ see, sysctl.d(5) manual pages.

6.2. Configuring kernel parameters temporarily with sysctl

The following procedure describes how to use the sysctl command to temporarily set kernel parameters at runtime. The command is also useful for listing and filtering tunables.

Prerequisites

  • Root permissions

Procedure

  1. To list all parameters and their values, use the following: 

    # sysctl -a
    Note

    The # sysctl -a command displays kernel parameters, which can be adjusted at runtime and at boot time.

  2. To configure a parameter temporarily, use the command as in the following example: 

    # sysctl <TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE>

    The sample command above changes the parameter value while the system is running. The changes take effect immediately, without a need for restart.

    Note

    The changes return back to default after your system reboots.

Additional resources

  • For more information about sysctl, see the sysctl(8) manual page.
  • To permanently modify kernel parameters, either use the sysctl command to write the values to the /etc/sysctl.conf file or make manual changes to the configuration files in the /etc/sysctl.d/ directory.

6.3. Configuring kernel parameters permanently with sysctl

The following procedure describes how to use the sysctl command to permanently set kernel parameters.

Prerequisites

  • Root permissions

Procedure

  1. To list all parameters, use the following: 

    # sysctl -a

    The command displays all kernel parameters that can be configured at runtime.

  2. To configure a parameter permanently: 

    # sysctl -w <TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE> >> /etc/sysctl.conf

    The sample command changes the tunable value and writes it to the /etc/sysctl.conf file, which overrides the default values of kernel parameters. The changes take effect immediately and persistently, without a need for restart.

Note

To permanently modify kernel parameters you can also make manual changes to the configuration files in the /etc/sysctl.d/ directory.

Additional resources

6.4. Using configuration files in /etc/sysctl.d/ to adjust kernel parameters

The following procedure describes how to manually modify configuration files in the /etc/sysctl.d/ directory to permanently set kernel parameters.

Prerequisites

  • Root permissions

Procedure

  1. Create a new configuration file in /etc/sysctl.d/: 

    # vim /etc/sysctl.d/<some_file.conf>

  2. Include kernel parameters, one per line, as follows: 

    <TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE>
<TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE>
  3. Save the configuration file.

  4. Reboot the machine for the changes to take effect.

    • Alternatively, to apply changes without rebooting, execute: 

      # sysctl -p /etc/sysctl.d/<some_file.conf>

      The command enables you to read values from the configuration file, which you created earlier.

Additional resources

  • For more information about sysctl, see the sysctl(8) manual page.
  • For more information about /etc/sysctl.d/, see the sysctl.d(5) manual page.

6.5. Configuring kernel parameters temporarily through /proc/sys/

The following procedure describes how to set kernel parameters temporarily through the files in the virtual file system /proc/sys/ directory.

Prerequisites

  • Root permissions

Procedure

  1. Identify a kernel parameter you want to configure: 

    # ls -l /proc/sys/<TUNABLE_CLASS>/

    The writable files returned by the command can be used to configure the kernel. The files with read-only permissions provide feedback on the current settings.

  2. Assign a target value to the kernel parameter: 

    # echo <TARGET_VALUE> > /proc/sys/<TUNABLE_CLASS>/<PARAMETER>

    The command makes configuration changes that will disappear once the system is restarted.

  3. Optionally, verify the value of the newly set kernel parameter: 

    # cat /proc/sys/<TUNABLE_CLASS>/<PARAMETER>

Additional resources