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

["Nick Couchman" <Nick.Couchman@xxxxxxxxx>]

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.

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.

-Nick

>>> On 2013/02/10 at 04:20, "Peter Gansterer"
<peter.gansterer@xxxxxxxxxxxxx>
wrote: 
> Hi again,
> 
> Are there really no hints about this topic?
> 
> It would also be useful to definitely know there is no solution for
what we 
> are looking for - or even input as to why this wouldn't work.
> Otherwise we'd think about trying it on our own.
> 
> - peda.
> 
> 
> On 2013-02-07 18:37:16.481416 Peter Gansterer wrote:
>> Hello,
>> 
>> In our office we would like to do something like "memory
overcommitment"
>> or rather "dynamic allocation" for a bunch of windows domUs (server
2003 & 
> 2008).
>> 
>> We thought this should be a simple feature, however we did not find
any
>> fitting solution.
>> We use Xen 4.2.1 with xl toolstack.
>> 
>> The idea is to use ballooning to dynamically (and automatically)
change
>> physical memory available to the domUs according to their memory
needs.
>> There was something called "xenballoond" for linux guests before
"tmem"
>> was integrated, and this seems to be exactly what we're looking
for,
>> except that we need it for windows domUs.
>> 
>> Is there really no such thing for windows guests, or are we just too
stupid 
> to find it?
>> We would be happy about any hints.
>> 
>> If there really is none, then, is there a reason for that?
>> 
>> It seems like a simple thing:
>> - start a process on each windows guest to monitor memory usage
>> - if there is spare memory -> tell the hypervisor to lower the
mem-target
>> - if memory is used up, decide upon CPU usage and mem history:
>>   . idle for some time -> try to free buffers by allocating and
freeing 
> blocks
>>   . loaded -> do nothing or increase mem-target, according to
pagefile usage
>>   . recently increased -> increase mem-target
>> - If memory is needed quickly, the pagefile will be used
temporarily.
>> - In a trivial implementation, a simple script on dom0 could
receive
>>   the mem-target requests and do the actual "xl mem-set" calls
>>   (according to some balancing algorithm).
>> 
>> Or is this approach just too naive?
>> 
>> In our scenario (virtual desktops and testservers) there are always
idle
>> guests wasting memory and not really using it. I suppose there must
be a
>> way to automatically (if just slowly) shift memory to those in need,
and
>> thus reducing overall memory requirements.
>> 
>> yours,
>> - peter.
>> 
>> -- 
>> Peter Gansterer
>> 
>> PARADIGMA Unternehmensberatung GmbH
>> Mariahilferstraße 47/1/3
>> A-1060 Wien
>> Tel: 0043-(0)1-585 49 72
>> http://www.paradigma.net
>> 
>> Firmenbuchnummer: FN 134564 p
>> Rechtsform: GmbH
>> Firmenbuchgericht: Handelsgericht Wien



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