Difference between logical and physical cpu hotplug

[Vladimir Murzin]

On Wed, Aug 17, 2011 at 9:14 AM, Vaibhav Jain <vjoss197 at gmail.com> wrote:
> 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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>
Hi Vaibhav!

I think physical CPU hot-plug is a capability to plug in and out
processing device (CPU) on running machine. Logical CPU hot-plug is a
capability to add on and remove out already available processing units
(CPU s or cores) under SMP domain of OS.

P.S. Sorry for my weak English.