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Chapter 5. Configuring kernel parameters at runtime

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As a system administrator, you can modify many facets of the Red Hat Enterprise Linux kernel’s behavior at runtime. 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.


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.

5.1. 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 5.1. Table of sysctl classes
Tunable classSubsystem


Execution domains and personalities


Cryptographic interfaces


Kernel debugging interfaces


Device-specific information


Global and specific file system tunables


Global kernel tunables


Network tunables


Sun Remote Procedure Call (NFS)


User Namespace limits


Tuning and management of memory, buffers, and cache

Additional resources

  • sysctl(8), and sysctl.d(5) manual pages

5.2. Configuring kernel parameters temporarily with sysctl

Use the sysctl command to temporarily set kernel parameters at runtime. The command is also useful for listing and filtering tunables.


  • Root permissions


  1. List all parameters and their values.

    # sysctl -a

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

  2. To configure a parameter temporarily, enter:


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


    The changes return back to default after your system reboots.

5.3. Configuring kernel parameters permanently with sysctl

Use the sysctl command to permanently set kernel parameters.


  • Root permissions


  1. List all parameters.

    # sysctl -a

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

  2. 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.


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

Additional resources

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

Modify configuration files in the /etc/sysctl.d/ directory manually to permanently set kernel parameters.


  • Root permissions


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

    # vim /etc/sysctl.d/<some_file.conf>
  2. Include kernel parameters, one per line.

  3. Save the configuration file.
  4. Reboot the machine for the changes to take effect.

    • Alternatively, to apply changes without rebooting, enter:

      # 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

  • sysctl(8), sysctl.d(5) manual pages

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

Set kernel parameters temporarily through the files in the /proc/sys/ virtual file system directory.


  • Root permissions


  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>

5.6. Additional resources

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