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

[Philipp Hahn <hahn@xxxxxxxxxxxxx>]

Hello Phil,

On Tuesday 15 May 2012 13:42:34 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?

At least from the Linux kernel I know that it needs memory to manage the 
memory itself. The data structures are scaled accoring to the maximum size, 
that is they are allocated at the very start to manange all that potential 
memory.
The actual memory is then reduced using ballooning, which doesn't affect those 
structures, since it only marks pages as unavailable to the OS, but those 
pages are still seen and managed by the OS, so the structure can't shrink.
Same's probably with Xen.

Sincerely
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn@xxxxxxxxxxxxx
Univention GmbH        be open.                       fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

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