Enable/Disable CPUs dynamically in Linux

Fri 09 May 2008 by Thejaswi Puthraya

Enable/Disable Multi CPUs in GNU/Linux

This is a wonderful feature of the Linux kernel. Being able to dynamically enable and disable multiple CPUs (all this without rebooting).

To check if you have multiple CPUs

$ cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 15
model name      : Intel(R) Core(TM)2 Duo CPU     T7100  @ 1.80GHz
stepping        : 13
cpu MHz         : 1795.675
cache size      : 2048 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 2
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 10
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov
                  pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe
                  nx lm constant_tsc arch_perfmon pebs bts pni monitor ds_cpl
                  vmx est tm2 ssse3 cx16 xtpr lahf_lm ida
bogomips        : 3596.78
clflush size    : 64

processor       : 1
vendor_id       : GenuineIntel
cpu family      : 6
model           : 15
model name      : Intel(R) Core(TM)2 Duo CPU     T7100  @ 1.80GHz
stepping        : 13
cpu MHz         : 1795.675
cache size      : 2048 KB
physical id     : 0
siblings        : 2
core id         : 1
cpu cores       : 2
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 10
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov
                  pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe
                  nx lm constant_tsc arch_perfmon pebs bts pni monitor ds_cpl
                  vmx est tm2 ssse3 cx16 xtpr lahf_lm ida
bogomips        : 3591.16
clflush size    : 64

So I have two cpus. Now to dynamically disable the second CPU.

# echo 0 > /sys/devices/system/cpu/cpuX/online

Here X is any number between 1 and the maximum number of CPUs. Observe the output of cpuinfo to check if the previous step worked.

To dynamically enable it,

# echo 1 > /sys/devices/system/cpu/cpuX/online

Note: Not all CPU architectures have support for this feature.

There are a lot of advantages of this feature:

Helps in benchmark tests (when you want to know the performance of your code over multiple and single cpus).
Helps you save that extra bit of power (esp on battery) in desparate situations.