Re: [Xen-users] "dynamic memory allocation" for windows guests?

["Peter Gansterer" <peter.gansterer@xxxxxxxxxxxxx>]

On 2013-02-10 16:28:47 Nick Couchman wrote:
> Peter,
> There are a couple of options.  First, I believe recent versions of Xen
> will let you set a maximum allocation and then a lower initial
> allocation (XCP at least lets you do this) and you can then allocate
> memory between the initial and maximum allocation.  Unfortunately, due
> to limitations with Windows virtualization, what actually happens is
> that Xen tells Windows that is has memory at the maximum level but marks
> the difference between the current allocation and the maximum allocation
> as used memory.  As you increase allocation up from the current
> allocation to your maximum allocation it frees up the memory in that
> range.  This requires a driver/daemon inside of Windows to accomplish
> the dynamic allocation - I think GPLPV drivers have support, and I know
> the XCP/XenServer drivers include support.

I am well aware of "ballooning" as a means to change the amount of memory 
allocated to a running guest. But you still have to manually set that memory 
target...

> Overcommitment is something entirely different and, while supported in
> the open source versions of the Xen hypervisor (look up xenpaging), is
> not at this time supported in XCP or commercial XenServer (to my
> knowledge).  Overcommitment is the idea of actually allocating more
> memory to guests than you have available on your physical machines, with
> the risk of running into a situation where you cannot fulfill the
> required commitment level to all of the guests.  Overcommitment is a
> hypervisor/dom0 feature and shouldn't require any additional drivers
> inside the guest to accomplish.

Now THAT is a good hint!
I wasn't aware of "xenpaging".
We will definitely give it a try, although there is a big fat Warning right on 
top of "xenpaging.txt" :-/

Thanks a lot!
- peter.

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