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Kernel boot parameters

How to configure kernel boot parameters with grub2

Example of the "isolcpus" option

  • Open the file /etc/default/grub. Example:
sudo gedit /etc/default/grub
  • Edit the option GRUB_CMDLINE_LINUX_DEFAULT in the file /etc/default/grub to boot all the kernels with the new option. In this case, the kernel boot option isolcpus=2,3 has been used, on a 4 cores CPU, to boot the kernel (and the other OS processes) using CPU cores 0 and 1, while CPU cores 2 and 3 are isolated, i.e., not used.
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash isolcpus=2,3"
  • Run the command:
sudo update-grub

to make active the GRUB configuration at the next reboot.

Choose which kernel use for boot

If you want to boot the third kernel of the second sub-menu, you have to modify in file /etc/default/grub the variable GRUB_DEFAULT in the following way:


and to make effective modification you have to run the command:

sudo update-grub

If you want to choose directly an entry of the main menu, for instance the third entry, the variable GRUB_DEFAULT has to be modified in the following way:


There is also the option to specify directly the name of the kernel (this is the solution I prefer):

GRUB_DEFAULT="Advanced options for Ubuntu>Ubuntu, with Linux 3.2.0-18-generic-pae"

To know the names of the kernels you can type the command:

grep menuentry /boot/grub/grub.cfg

Boot in recovery mode

  • Select Recovery mode among kernel options
  • Select a root section
  • Type mount -o remount,rw /, if you want to enable write access to the filesystem

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