Multicore CPU: how to disable a core

For overclocking adepts it's quite useful to be able to determine whether a core is working properly during rise in frequencies or for testing purposes (e.g. verify if in your Quad Core a core is damaged or failing, thus reducing overall performance).

Sometimes this procedure is necessary to run certain applications, which are not compatible with multi core processing.

Note that this procedure will not affect physically your hardware, but consider that your OS (Windows or Linux) will simply ignore the core(s) you selected.

Procedure when using Windows (Vista, 7 and XP)

• Open the start menu, click on Run,type in msconfig and click on OK
• In the System Configuration window click on the start sub menu and the on Advanced Options button.

• In the second window that appear, you should be provide with an option for enabling and disabling the number of processors you want to use.
• Validate your entry by clicking on OK.
• Restart your PC and it should be fine.

Procedure when using Linux:

When using Linux the procedure is quite different, when starting up your PC, you should find the Grub menu, which is responsible for launching Linux. Select the entry you want to use, press e key and a menu will appear (depending on the version used).

title         Ubuntu 7.10, kernel 2.6.22-14-generic
root         (hd0,5)
kernel       /boot/vmlinuz-2.6.22-14-generic root=UUID=a8a39729-9d2a-425b-b84b-ecc5c7dad104 ro quiet splash
initrd         /boot/initrd.img-2.6.22-14-generic
quiet

Using the arrow key, go the line beginning with the word kernel and press on “e” again.

Be aware that you are editing the line, which defines the path to your Linux kernel (core), at the end of the line add the following command:

maxcpus=1

The end result should be:

kernel  /boot/vmlinuz-2.6.22-14-generic root=UUID=a8a39729-9d2a-425b-b84b-ecc5c7dad104 ro quiet maxcpus=1

Note that que ro, quiet and maxcpus are the parameters send to the core when starting up Linux.

Press on the b key to start Linux. Only one of your processor should be used by Linux, the rest will be ignored.