Trouble enabling hyperthreading on i7 (enabled in BIOS)

thrice · 06-05-2019, 02:51 AM

Hi everyone,

I just installed a fresh copy Xubuntu 18.04 and I've noticed that hyperthreading is not getting enabled for some reason, and I'm not sure why. I have an Intel i7 6950X with 10 cores and HT support - here's a snippet of output from lscpu, which confirms only 1 thread per core:

Code:

$ lscpu
Architecture:        x86_64
CPU op-mode(s):      32-bit, 64-bit
Byte Order:          Little Endian
CPU(s):              10
On-line CPU(s) list: 0-9
Thread(s) per core:  1
Core(s) per socket:  10
Socket(s):           1
NUMA node(s):        1
Vendor ID:           GenuineIntel
CPU family:          6
Model:               79
Model name:          Intel(R) Core(TM) i7-6950X CPU @ 3.00GHz

HT support shows up as being supported, however:

Code:

$ sudo grep -o '^flags\b.*: .*\bht\b' /proc/cpuinfo | tail -1
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht

I've checked my BIOS and hyperthreading is enabled, and there are no issues with hyperthreading in my Windows 10 install with the current BIOS settings.

I came across this thread on stackexchange, and one of the posts mentioned that it's possible to check if HT is enabled with "cat /sys/devices/system/cpu/smt/active", and switch it on and off with
"echo [on|off] > /sys/devices/system/cpu/smt/control". The result of the first command confirms that it's disabled, but the location /sys/devices/system/cpu/smt doesn't exist in my install. Any ideas?

Thanks in advance.

hydrurga · 06-05-2019, 04:26 AM

Quote:

Originally Posted by thrice

The result of the first command confirms that it's disabled, but the location /sys/devices/system/cpu/smt doesn't exist in my install. Any ideas?

Yes. Create the file by using the suggested command.

dc.901 · 06-05-2019, 06:17 AM

This may help explain: https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/L1TF

jefro · 06-05-2019, 02:58 PM

Interesting info dc.901

Seems that many places are suggesting that HT be disabled. Not sure if the distro and version went ahead and disabled by default but kind of looks like it.

Try some live boot of an older distro to double check. Even a Knoppix should show penguins at boot.