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How to Hot plug CPU in RHEL 5


Red Hat Enterprise Linux 5 supports the cpu-hotplug mechanism, which allows for CPUs to be dynamically disabled and re-enabled on a system without requiring a system reboot. Note: CPU hot-plugging will only work if physical hotplug is supported by the hardware. A more novel use of CPU-hotplug support is its use in suspend resume support for SMP. Dual-core and HT support makes even a laptop run SMP kernels which didn’t support these methods.

In order to disable a CPU core in a running machine, use the file /sys/devices/system/cpu/cpuX/online.

The following command will disable a CPU:

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

Where X is the ID of the CPU as determined from /proc/cpuinfo.

To re-enable the CPU, run:

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


The following output from /proc/cpuinfo shows two processors on a system:

root@xen # grep “processor” /proc/cpuinfo
processor       : 0
processor       : 1

In order to disable CPU 1, echo “0” to /sys/devices/system/cpu/cpu1/online:

root@xen# echo 0 > /sys/devices/system/cpu/cpu1/online
cpu0/ cpu1/

Check /proc/cpuinfo to see if the cpu is disabled .

root@xen # grep “processor” /proc/cpuinfo
processor       : 0

Notice that there is now only one processor shown (CPU 0) instead of two.

Note: There are some architectures in which a CPU cannot be disabled due to a dependency on a certain CPU. In such cases you will notice that the /sys/devices/system/cpu/cpuN/online file is missing.

For further details on CPU hotplugging, see this file:


The documentation files are in the kernel-doc package, which can be installed from RHN.



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