How to disable core(s) of CPU

Introduction ~

The question is why we  need to disable few core of CPU? Sometime it is necessary to run certain applications, which are not compatible with multi core processing.  Disabling core will not affect physically your hardware. Linux OS will simply ignore the core(s) you selected to disable.

Steps 1] How to do it?

Debian/Ubuntu ~

root@laptop:/home/arunsb# cat /boot/grub/menu.lst

title        Ubuntu 9.04, kernel 2.6.28-11-generic
kernel        /vmlinuz-2.6.28-11-generic root=UUID=55d33e45-75c7-54sc-b204-97b44e1d6a39 ro quiet splash maxcpus=1
initrd        /initrd.img-2.6.28-11-generic

Redhat/Fedora based system ~

root@laptop:/home/arunsb# cat /boot/grub/grub.conf

title Red Hat Enterprise Linux ES (2.6.9-78.ELsmp)
root (hd0,4)
kernel /boot/vmlinuz-2.6.9-78.ELsmp ro root=LABEL=/    maxcpus=1
initrd /boot/initrd-2.6.9-78.ELsmp.img

Note ~ after changing grub config file please reboot the system to apply changes!

As shown above “maxcpus=1” indicates that Linux will use only one CPU core. you can change this value as per your requirement and hardware available.

You can  also change this value during  starting of system from GRUB menu but it is temporary setting. To make it permanent you need to modify the  grub.conf (Redhat/Fedora) or menu.lst (debian/Ubuntu) GRUB config file.

Step 2] How to verify ~

I have dual core CPU as shown below and I have disable 1 core so After reboot I should get only one CPU core active

** Before above setting!

root@laptop:/home/arunsb# cat /proc/cpuinfo   | grep processor
processor    : 0
processor    : 1
root@laptop:/home/arunsb#

* Verify after above setting ~

root@laptop:/home/arunsb# cat /proc/cpuinfo   | grep processor
processor       : 0
root@laptop:/home/arunsb#

* How to Disable CPU without Reboot?
root@arunb:~# echo 0 > /sys/devices/system/cpu/cpu1/online

* Confirm ?
root@arunb:~# cat /proc/cpuinfo | grep -i ‘Processor’
processor : 0
root@arunb:~#

Thank you,
Arun Bagul

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