problems with understanding of the memory parameters in the xml file

Peter Krempa pkrempa at
Wed Feb 12 07:34:06 UTC 2020

On Tue, Feb 11, 2020 at 18:48:43 +0100, Lentes, Bernd wrote:
> Hi guys,

Hi Bernd,

> despite reading hours and hours in the internet i'm still struggling with
> "memory", "currentmemory" and "maxMemory".

to briefly summarize what those three knobs do:

1) memory - this is the initial memory size for a VM. qemu grants this
amount of memory to the VM on start. This is also the memory the guest
is able to use if the balloon driver is not loaded as the balloon driver
voluntarily gives up memory from the guest OS to the host.

2) currentmemory - in case the guest is using the balloon driver this is
the actual memory size it's using. This field is dynamically updated to
reflect the size reported by the balloon driver

3) maxMemory - This knob controls the maximum size of the memory when
memory hotplug is used. This basically sets the amount of address space
and memory slots the VM has so that new memory can be plugged in later.

> Maybe you can help me to sort it out.
> My idea is that a guest has an initial value of memory (which "memory" seems to be) when booting.

Yes, until the memballoon driver is loaded the guest can use 'memory' +
any hotpluggable memory modules.

> We have some Windows 10 guests which calculate some stuff and i would like to increase memory during runtime
> until it reaches a fixed maximum value.
> My hope was that a higher "maxMemory" could solve this, that the guest claims more memory and gets it.
> I didn't get it. Is my idea wrong ? Do i need a balloon driver for that ?

No this is for memory hotplug and doesn't work automatically. That
configuration knob just sets the size and number of slots.

You then have to add a entry into the <devices> section:

<memory model='dimm'>
    <size unit='MiB'>2048</size>

The above can also be added during runtime e.g. using virsh
attach-device. Hence hotplug. It can also be unplugged during runtime
but that requires guest cooperation and there are a few caveats of this.
Namely to successfully unplug the memory the guest must not write any
non-movable pages into it so that it can give up the memory later. On
linux that means that no memory-mapped I/O regions can be created there
which may lead to weird guest behaviour if the memory is onlined as
movable. I'm not sure how windows behaves in this regard though, but
AFAIK it supports memory hotplug just fine.

> What i find concerning ballooning is that it doesn't work automatically but has to be adjusted
> manually. Is that right ?

No, unfortunately none of this works automatically.

> Balloon drivers for windows are available.
> Is my idea right, does that work basically ? If yes how do i have to set the parameters ?
> Is the memory released after the guest has e.g. finished his calculation ?
> Does that work automatically or do i have to adjust that manually ?

When using the balloon driver you can set the 'currentMemory' size down
to some reasonable value and the balloon driver will then return the
memory to the host. There were some attempts to make this automatic, but
I don't remember how they went. One other caveat is that any memory
returned by the balloon driver to the host may be available to the guest
again e.g. on reboot when the balloon driver is removed.

For a 1 NUMA node guest the memory hotplug an balloon can theoretically
be combined but unplugging of the memory might not work while the ballon
is inflated.

I hope this clarified it somehwat.

> Thanks.
> Bernd
> -- 
> Bernd Lentes 
> Systemadministration 
> Institute for Metabolism and Cell Death (MCD) 
> Building 35.34 - Raum 208 
> HelmholtzZentrum münchen 
> bernd.lentes at 
> phone: +49 89 3187 1241 
> phone: +49 89 3187 3827 
> fax: +49 89 3187 2294 
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