How to disable or enable hyper threading on linux?

Hyper threading uses processor resources more efficiently, enabling multiple threads to run

on each core. As a performance feature, it also increases processor throughput, improving

overall performance on threaded software. A single physical CPU core with hyper-threading

appears as two logical CPUs to an operating system.

The recommended way to disable HT is by disabling in the BIOS, if possible but this can

also be done via operating system using the below steps.

Disable HT on runtime for individual logical CPUs

Before starting let's check the lscpu stat

# lscpu

Architecture:          x86_64

CPU op-mode(s):        32-bit, 64-bit

Byte Order:            Little Endian

CPU(s):                32

On-line CPU(s) list:   0-31

Thread(s) per core:    2

Core(s) per socket:    8

Socket(s):             2

Here it shows that there are 2 threads per core so we know most likely hyper threading is

enabled

The following files will show all of the logical CPU's and their HT pair relationships

# grep -H . /sys/devices/system/cpu/cpu*/topology/thread_siblings_list

To determine which CPUs should be disabled, the threads running on the same CPU core

have to be identified. The files /sys/devices/system/cpu/cpuN/topology/thread_siblings_list

where N is the CPU socket number. This file will contain the logical (HT) CPU numbers for

each physical socket.

# grep -H . /sys/devices/system/cpu/cpu*/topology/thread_siblings_list | sort -n -t ',' -k 2 -u

/sys/devices/system/cpu/cpu0/topology/thread_siblings_list:0,16

/sys/devices/system/cpu/cpu17/topology/thread_siblings_list:1,17

/sys/devices/system/cpu/cpu18/topology/thread_siblings_list:2,18

/sys/devices/system/cpu/cpu19/topology/thread_siblings_list:3,19

/sys/devices/system/cpu/cpu20/topology/thread_siblings_list:4,20

/sys/devices/system/cpu/cpu21/topology/thread_siblings_list:5,21

/sys/devices/system/cpu/cpu22/topology/thread_siblings_list:6,22

/sys/devices/system/cpu/cpu23/topology/thread_siblings_list:7,23

/sys/devices/system/cpu/cpu24/topology/thread_siblings_list:8,24

/sys/devices/system/cpu/cpu25/topology/thread_siblings_list:9,25

/sys/devices/system/cpu/cpu10/topology/thread_siblings_list:10,26

/sys/devices/system/cpu/cpu11/topology/thread_siblings_list:11,27

/sys/devices/system/cpu/cpu12/topology/thread_siblings_list:12,28

/sys/devices/system/cpu/cpu13/topology/thread_siblings_list:13,29

/sys/devices/system/cpu/cpu14/topology/thread_siblings_list:14,30

/sys/devices/system/cpu/cpu15/topology/thread_siblings_list:15,31

This means that CPU0 and CPU16 are threads on the same core. The same for 1 and 17

and so on. Individual, logical HT CPUs could be turned off as needed for a specific

application that is bound to a physical core.

Or the following script would disable all of them, from logical CPU 16 through 31

# cat /tmp/disable_ht.sh

#!/bin/bash

for i in {16..31}; do

   echo "Disabling logical HT core $i."

   echo 0 > /sys/devices/system/cpu/cpu${i}/online;

done

To disable individual logical CPU use the below command and replace <cpu_id> with the id from (16..31)

echo 0 > /sys/devices/system/cpu/<cpu_id>/online

To re-enable the HT

# cat /tmp/enable_ht.sh

for i in {16..31}; do

   echo "Enabling logical HT core $i."

   echo 1 > /sys/devices/system/cpu/cpu${i}/online;

done