Difference between logical and physical cpu hotplug

[Vaibhav Jain]

On Thu, Aug 18, 2011 at 9:02 AM, srivatsa bhat <bhat.srivatsa at gmail.com>wrote:

> Hi Vaibhav,
>
> On Thu, Aug 18, 2011 at 8:24 PM, Vaibhav Jain <vjoss197 at gmail.com> wrote:
>
>> Hi,
>>
>> I talked to a friend of mine and he suggested that
>> in a logical offline state the cpu is powered on and ready to execute
>> instructions
>> just that the kernel is not aware of it. But in case of physical offline
>> state the cpu
>> is powered off and cannot run.
>> Are you saying something similar ?
>>
>> Yes, you are right, mostly.
> When you try to logically offline a CPU, the kernel will do task migration
> (i.e., move out all the tasks running on that CPU to other CPUs in the
> system) and it ensures that it doesn't need that CPU anymore. This also
> means that, from now on, the context of that CPU need not be saved (because
> the kernel has moved that CPU's tasks elsewhere). At this point, it is as if
> the kernel is purposefully using only a subset of the available CPUs. This
> step is a necessary prerequisite to do physical CPU offline later on.
>
> But I don't think CPU power ON or OFF is the differentiating factor between
> logical and physical offlining. In logical offline, you still have the CPUs
> in the system but you just tell the kernel not to use them. At this stage,
> you can power off your CPU, to save power for example.
> But in physical offline, from a software perspective, you do additional
> work at the firmware level (apart from logical offlining at the OS level),
> to ensure that physically plugging out the CPUs will not affect the running
> system in any way.
>
> Please note that you can logically online and offline the same CPUs over
> and over again without rebooting the system. Here, while onlining a CPU
> which was offlined previously, the kernel follows almost the same sequence
> which it normally follows while booting the CPUs during full system booting.
>
> Also one more thing to be noted is that, to be able to physically hot-plug
> CPUs, apart from OS and firmware support, you also need the hardware to
> support this feature. That is, the electrical wiring to the individual CPUs
> should be such that plugging them in and out does not interfere with the
> functioning of the rest of the system. As of today, there are only a few
> systems that support physical CPU-hotplug. But you can do logical CPU
> hotplug easily, by configuring the kernel appropriately during compilation,
> as you have noted in one of your previous mails.
>
> Regards,
> Srivatsa S. Bhat
>


Hi Srivatsa,

That was great explanation! Thanks!
I have just one more query. You mentioned above that " the kernel follows
almost the same *sequence *which it normally follows while booting the CPUs
during full system booting."

Can you please explain this sequence a little ?

Thanks
Vaibhav Jain
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