Hyperthreading on 32-bit but not on 64-bit?

Question

I have a strange situation. I have two computers with identical hardware, both Core i5, exactly the same model. The only difference is the OS, one has 12.04 32-bit and the other one has 12.04 64-bit.

The problem is that hyperthreading is only enabled on the 32-bit OS and not on the 64-bit OS. Can you tell me why, and how I can enable it?

I looked in the BIOS and both have exactly the same settings, both hyperthreading enabled in BIOS but not in the OS.

2 logical processors:

 Architecture:          x86_64  
CPU op-mode(s):        32-bit, 64-bit  
Byte Order:            Little Endian  
CPU(s):                2  
On-line CPU(s) list:   0,1  
Thread(s) per core:    1  
Core(s) per socket:    2  
Socket(s):             1  
NUMA node(s):          1  
Vendor ID:             GenuineIntel  
CPU family:            6  
Model:                 42  
Stepping:              7  
CPU MHz:               2294.665  
BogoMIPS:              4589.73  
Virtualization:        VT-x  
L1d cache:             32K   
L1i cache:             32K  
L2 cache:              256K  
L3 cache:              3072K  
NUMA node0 CPU(s):     0,1 

4 logical processors:

Architecture:          i686  
CPU op-mode(s):        32-bit, 64-bit  
Byte Order:            Little Endian  
CPU(s):                4  
On-line CPU(s) list:   0-3  
Thread(s) per core:    2  
Core(s) per socket:    2  
Socket(s):             1  
Vendor ID:             GenuineIntel  
CPU family:            6  
Model:                 42  
Stepping:              7  
CPU MHz:               800.000  
BogoMIPS:              4589.36  
Virtualization:        VT-x  
L1d cache:             32K  
L1i cache:             32K  
L2 cache:              256K  
L3 cache:              3072K  

Answer 1

Make sure you don't boot with acpi=off as this disables hyperthreading. You can boot with acpi=ht, which is just like acpi=off, but enables just the required parts of acpi to use hyperthreading.

Answer 2

There are plenty of CPUs under Core i5 name. take a look at this link to see a complete list of them.

There are core i5 CPUs with 2 cores (codename Clarkdale and Arrandale) and 4 cores (codename Lynnfield) for more information see this link.

Answer 3

When you need to know whether hyper-threading is enabled without the need to reboot the system (and consulting the BIOS), you can simply look at the output of cat /proc/cpuinfo and compare the siblings with the CPU cores fields.

Even though cat /proc/cpuinfo shows you all the logical CPUs (processor field) in the system, the siblings field holds the number of logical CPUs for the physical CPU this entry belongs to (including both the cores and the hyper-threaded LCPUs).

In other words, if you see:

processor : 8
physical id : 9
siblings : 4
cpu cores : 2

It means that LCPU #8 (the eight logical CPU in your system) is one of the 4 logical CPUs on the physical CPU that has 2 cores. So yes, hyper-threading is enabled.

If the number of CPU cores is the same as the number of siblings, hyper-threading is disabled.