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On a MacBookPro 7,1 running ubuntu 11.10, System monitor only sees 1 core (instead of 2).

cat /proc/cpuinfo | grep 'cpu cores'

also gives:

cpu cores   : 1

I followed this guide and added acpi_apic_instance=2 to the line with GRUB_CMDLINE_LINUX_DEFAULT, but that doesn't seem to change the situation.

What can I do? I'm using rEFIt, installed under MacOS, and running in dual boot with MacOS. After the rEFIT menu, I'm still presented with the GRUB menu (I'm assuming that's normal).

I saw similar posts on this matter, but could not fix my problem with what they suggested.

EDIT: With the method mentioned above the computer runs sometimes with 1 core and other times with 2. Why is that? How can it be fixed?

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OP are you still looking for an answer? If so, you may need to repost your question as this is flagged for closure. Regards, – Ringtail Mar 17 '12 at 18:12
Yes, I'm still looking for an answer unfortunately. – gozzilli Mar 20 '12 at 9:13
Please EDIT your QUESTION and remove your ANSWER as to what you have done. I recommend Booting from a Live CD/DVD and retest. – Ringtail Mar 20 '12 at 19:06
Links to the other questions you've tried might help here too. – Jorge Castro Mar 21 '12 at 23:27

I have a bit older macbook 15" (model name 5.2) And all works fine. Ive spent some effort on getting it to work in EFI mode though.

Are you booting your mac in EFI mode , or in the old "legacy mode"?

You can try to efi boot your mac from a ubuntu livecd/ USB (you will be presented with both efi boot ubuntu or "legacy"

check from there if both cpu:s are detected

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