My Dell Optiplex 980 at work doesn’t seem to play nice with Ubuntu 12.04 LTS. It fails to detect hyper-treading support on the CPU and defaults back to a single core setup when effectively running a 8-core CPU. This is rather annoying.
It seems that acpi settings are to blame, adding:
to your grub config file mediates this issue and enables SMP support. Sadly, adding this parameter is undone by kernel updates.