This was a weird problem that I faced when I installed Fedora 12 on my Intel 2140 Dual Core pc. The installtion went fine and Linux was working properly. But I discovered that Linux did not actually detect second CPU core when I examined /proc/cpuinfo. Earler I had used Fedora 10 which detected both the cores properly and worked well.

After doing some googling in the Internet, I found that Fedora 12 had disabled SMP mode during boot up and the problem can be rectified by placing “noapic acpi=off” in the Linux kernel command line options. Hmm.. The solution did work. However I did not know the exact background of the problem though.

If you don’t know how to add the above said option in Linux kernel command line, just open /etc/grub.conf file and search for the first line similar to below. This assumes that you have “default=0″ in a line in the file.

kernel /vmlinuz-2.6.27.21-170.2.56.fc10.i686 ro root=UUID=852ffb43-43c5-8af4-c829-b2a304052cae0 rhgb

Now add “noapic acpi=off” to that as shown below.

kernel /vmlinuz-2.6.27.21-170.2.56.fc10.i686 ro root=UUID=852ffb43-43c5-8af4-c829-b2a304052cae0 rhgb noapic acpi=off

If you don’t find a file named /etc/grub.conf, then look for /boot/grub/menu.lst file.