Chapter 5. Configuring kernel parameters at runtime

Procedure

1. List all parameters and their values.

   # 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:

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

   This sample changes the parameter value while the system is running. The changes take effect immediately and it does not require system reboot.

   Note

   The changes return back to default after your system reboots.

Procedure

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.

   Note

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

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:

   <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:

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

     With this command, you can read values from the configuration file which you created earlier.

     See sysctl(8) and sysctl.d(5) man pages on your system for more information.

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 configuration changes applied by using a command are not permanent and will disappear after system reboot.

Verification

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

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