Understanding Linux Kernel Parameters

Introduction to Kernel Parameters

Kernel parameters, also known as kernel boot parameters or kernel command line parameters, are options passed to the Linux kernel at boot time. These parameters can modify kernel behavior, enable or disable features, and provide configuration information.

Pro Tip: Kernel parameters can be temporary (set at boot time) or permanent (set in the bootloader configuration).

Where to Set Kernel Parameters

GRUB2 Configuration: Edit /etc/default/grub and update the GRUB_CMDLINE_LINUX line.
GRUB Menu: Press 'e' at the GRUB menu to edit parameters for a single boot.
sysctl: Use sysctl to modify kernel parameters at runtime (some parameters only).
/proc/cmdline: View current kernel parameters.

Common Kernel Parameters

Parameter	Description	Example
root=	Specifies the root filesystem	`root=/dev/sda1`
init=	Specifies the init process	`init=/bin/bash`
quiet	Reduces verbosity of kernel messages	`quiet`
splash	Enables the boot splash screen	`splash`
mem=	Sets the amount of memory the kernel can use	`mem=4G`
nomodeset	Disables kernel mode setting	`nomodeset`

Advanced Kernel Parameters

acpi=off: Disables ACPI (Advanced Configuration and Power Interface)
noapic: Disables Advanced Programmable Interrupt Controller
noapm: Disables Advanced Power Management
vga=: Sets the VGA mode for console
elevator=: Selects I/O scheduler (e.g., elevator=deadline)
pci=nomsi: Disables Message Signaled Interrupts for PCI devices

Kernel Parameters for Troubleshooting

single or 1: Boots into single-user mode
emergency: Boots into emergency mode (minimal environment)
debug: Enables kernel debugging output
ignore_loglevel: Prints all kernel messages to console
nosmp: Disables SMP (Symmetric Multi-Processing)
maxcpus=: Limits the number of CPUs used

Modifying Kernel Parameters

Temporary Changes

Reboot your system and access the GRUB menu (usually by pressing Shift or Esc during boot)
Select the kernel you want to boot and press 'e' to edit
Find the line starting with 'linux' and add your parameters at the end
Press Ctrl+X or F10 to boot with these parameters

Permanent Changes

Edit the GRUB configuration file: sudo nano /etc/default/grub
Modify the GRUB_CMDLINE_LINUX line to add your parameters
Save the file and exit the editor
Update GRUB: sudo update-grub (Ubuntu/Debian) or sudo grub2-mkconfig -o /boot/grub2/grub.cfg (CentOS/RHEL)
Reboot your system for changes to take effect

Best Practices

Always backup your system before making changes to kernel parameters
Test temporary changes before making them permanent
Document any changes you make to kernel parameters
Be cautious when using parameters that affect hardware functionality
Consult distribution-specific documentation for any unique parameters