Re: [Xen-users] Missing memory when using maxmem

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-05-15 at 12:50 +0100, Phil Evans wrote:
> I am trying to allow domains' memory allocation to be altered by using
> both the "memory" and "maxmem" options in my config files (i.e. set
> maxmem to some large value to allow the domain to be resized up to
> that amount, and memory to set the initial allocation). 
> 
> However I appear to lose potentially quite a large chunk of memory as
> I use a larger "maxmem" option.  For instance if I use memory=512, as
> I use a higher value for maxmem, I lose more memory according to
> "free" in the guest.  I lose about 200MB (nearly half the memory!) if
> I have a maxmem of 8GB and even more when it is higher. 
> 
> I have looked around and couldn't find any mention of maxmem taking a
> chunk out of the memory by design.  Would anybody be able to shed any
> light on this? 

When the guest kernel starts up it needs to size various internal data
structures (e.g. the frame table, various page table data structures
etc) according to the maximum size it expects ever to see (so maxmem).

There is a per-guest limit onto how small you can make memory compared
to maxmem.

More modern pvops Linux kernels support memory hotplug which does away
with some of this limitation, although I'm not sure if tools support
exists.

Ian.



_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxx
http://lists.xen.org/xen-users