Difference between logical and physical cpu hotplug

[Vaibhav Jain]

Hi,

I need to know the difference between making a cpu logically offline
and physically offline. I have read that the following command

$ echo 0  > /sys/device/system/cpu/<cpu number>/online

makes a cpu logically offline. It frees the cpu from interrupts and migrates
running processes.
But then what does it mean to make a cpu physically offline ?
I referred to following article :

http://www.kernel.org/doc/Documentation/cpu-hotplug.txt

Here is the relevant excerpt :

Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
A: The usage of hot-add/remove may not be very consistently used in the code.
CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
To support physical addition/removal, one would need some BIOS hooks and
the platform should have something like an attention button in PCI hotplug.
CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.



Please help!

Thanks
Vaibhav Jain
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