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 class Subsystem 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 List all parameters and their values. Note The # sysctl -a command displays kernel parameters, which can be adjusted at runtime and at boot time. 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. Note The changes return back to default after your system reboots. Additional resources The sysctl(8) manual page Using configuration files in /etc/sysctl.d/ to adjust kernel parameters 5.3. Configuring kernel parameters permanently with sysctl Use the sysctl command to permanently set kernel parameters. Prerequisites Root permissions Procedure List all parameters. The command displays all kernel parameters that can be configured at runtime. Configure a parameter permanently: 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 The sysctl(8) and sysctl.conf(5) manual pages Using configuration files in /etc/sysctl.d/ to adjust kernel parameters 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 Create a new configuration file in /etc/sysctl.d/ : Include kernel parameters, one per line: Save the configuration file. Reboot the machine for the changes to take effect. Alternatively, apply changes without rebooting: 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 Identify a kernel parameter you want to configure. 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. Assign a target value to the kernel parameter. The configuration changes applied by using a command are not permanent and will disappear once the system is restarted. Verification Verify the value of the newly set kernel parameter.

Commands:
sysctl -a
sysctl <TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE>
sysctl -a
sysctl -w <TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE> >> /etc/sysctl.conf
vim /etc/sysctl.d/< some_file.conf >
< TUNABLE_CLASS >.< PARAMETER >=< TARGET_VALUE > < TUNABLE_CLASS >.< PARAMETER >=< TARGET_VALUE >
sysctl -p /etc/sysctl.d/< some_file.conf >
ls -l /proc/sys/< TUNABLE_CLASS >/
echo < TARGET_VALUE > > /proc/sys/< TUNABLE_CLASS >/< PARAMETER >
cat /proc/sys/< TUNABLE_CLASS >/< PARAMETER >