Re: [Xen-devel] Usage and details for XEN_BALLOON_MEMORY_HOTPLUG

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Mon, Jul 15, 2013 at 01:50:33AM +0800, Gmail wrote:
> Hi there,
> I have noticed that there is XEN-BALLON-MEMORY_HOTPLUG config since Linux 3.1.
> IMO, this is a module for hypervisor to allocate more memory than VM start-up
> configuration from the limited description in kernel codes. I have some 
> questions

No, this is guest module but maybe in the future it would
be possible use it in dom0. Now it does not work well in dom0.
There is also special module to support memory hotplug in
physical machine under Xen but I have not tested it yet.

> about the usage and details of this config:
>
> 1. Does this module support memory reclame when guest decrease
> its memory usage? If so, what is the strategy? Or just do it manually?

It supports memory reclaim by standard ballooning mechanism.
Memory hot remove is not supported. You must request
ballooning down manually (using xl command or internally
from guest; you could find more details in help for
relevant kernel config option).

> 2. What about handle guest memory only by hotplug? Is there any mechanism
> to support inflating/deflating guest memory simply by memory hotplug?

No.

> 3. How about the overhead of reconstructing the MMU related area when
> executing memory hotplug? Are there any articles refer to this?

I did not dig very deep but I have not seen anything.
I suppose that it is quite expensive and that is why
memory hotplug is not very well suited to do things
similar to ballooning down in particular (inter alia).

> Besides, does Linux support removing physical memory now
> (in physical environment, not VM)?

Yes, it does. IIRC, you should look for memory hot remove option.

Daniel

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