Re: CPU hot-plug and RAM hot-add in virtual machines

From: Andriy Gapon <>
Date: Wed, 21 Jul 2021 10:45:52 UTC
On 21/07/2021 11:53, Miroslav Lachman wrote:
> On 09/07/2021 13:16, Andriy Gapon wrote:
>> On 08/07/2021 00:34, Miroslav Lachman wrote:
>>> The question is simple but I cannot find answer - Does FreeBSD support 
>>> hot-plug vCPU and hot-add RAM?
>>> Current virtualization platforms support adding CPU cores or additional RAM 
>>> without the need to reboot the guest OS. Some of our clients need to add 
>>> additional vCPUs or RAM so often that hot-plug and hot-add will be really 
>>> useful. If this is not supported on FreeBSD for now, is there any Work In 
>>> Progress? Or is there a plan to support it?
>> I think that those features are not supported and I haven't heard of any WIP.
> Thank you for your reply. I know nothing about system internals. Is it too much 
> work to implement it or just nobody from developers need it thus nobody write 
> the code? Virtualization is more and more popular these days so I think it will 
> be useful for many users.

In my opinion, both would be big changes.
At the moment we do not have any support for offlining / onlining CPUs and that 
would involve a lot of careful changes like safely updating CPU sets, various 
CPU masks, etc.

Ditto for the memory.  Updating page arrays, free memory accounting structures, 
possibly some memory maps, etc.

Definitely not a weekend project.

Andriy Gapon