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Chapter 5. 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. 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.

Important

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

For more information about tuning kernel on IBM DB2, see Tuning Red Hat Enterprise Linux for IBM DB2.

5.1. What are kernel parameters

Kernel parameters are tunable values that you can adjust while the system is running. Note that for changes to take effect, you do not need to reboot the system or recompile the kernel.

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

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

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.

Prerequisites

  • Root permissions

Procedure

  1. List all parameters and their values.

    Copy to Clipboard Toggle word wrap 
    # 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, enter:

    Copy to Clipboard Toggle word wrap 
    # 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

5.3. Configuring kernel parameters permanently with sysctl

Use the sysctl command to permanently set kernel parameters.

Prerequisites

  • Root permissions

Procedure

  1. List all parameters.

    Copy to Clipboard Toggle word wrap 
    # sysctl -a

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

  2. Configure a parameter permanently:

    Copy to Clipboard Toggle word wrap 
    # 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

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

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

Prerequisites

  • You have root permissions.

Procedure

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

    Copy to Clipboard Toggle word wrap 
    # vim /etc/sysctl.d/<some_file.conf>

  2. Include kernel parameters, one per line:

    Copy to Clipboard Toggle word wrap 
    <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, apply changes without rebooting:

      Copy to Clipboard Toggle word wrap 
      # 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.

Prerequisites

  • Root permissions

Procedure

  1. Identify a kernel parameter you want to configure.

    Copy to Clipboard Toggle word wrap 
    # 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.

    Copy to Clipboard Toggle word wrap 
    # echo <TARGET_VALUE> > /proc/sys/<TUNABLE_CLASS>/<PARAMETER>

    The configuration changes applied by using a command are not permanent and will disappear once the system is restarted.

Verification

  1. Verify the value of the newly set kernel parameter.

    Copy to Clipboard Toggle word wrap 
    # cat /proc/sys/<TUNABLE_CLASS>/<PARAMETER>
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