[libvirt-users] no callback on VIR_DOMAIN_EVENT_ID_BALLOON_CHANGE in 0.10.2

Ben Clay rbclay at ncsu.edu
Wed Oct 3 23:04:10 UTC 2012


Thanks for the quick response.  I'm glad to hear about the rate-limiting,
that would've stumped me once I got it up and running. I'll give qemu 1.2 a
shot, and switch to a newer distro if needed.

Ben Clay
rbclay at ncsu.edu

-----Original Message-----
From: Eric Blake [mailto:eblake at redhat.com] 
Sent: Wednesday, October 03, 2012 4:42 PM
To: Ben Clay
Cc: libvirt-users at redhat.com
Subject: Re: [libvirt-users] no callback on

On 10/03/2012 04:08 PM, Ben Clay wrote:
> I'm trying to track balloon growth after issuing a setmem command to a 
> KVM guest with libvirt 0.10.2 and qemu-kvm on CentOS 6.3.  
> libvirt
> 0.10.2 was built from tar today and appears to be working fine.  The 
> guest is running CentOS 6.3 as well.

> Is my understanding incorrect of what VIR_DOMAIN_EVENT_ID_BALLOON_CHANGE

This event is only fired if you have qemu 1.2 or newer, or if the new
BALLOON_CHANGE event has been backported to the particular qemu you are

> Is there something else this event is tracking, other than a virsh -c 
> qemu:///system setmem <dom> <mem in KB> command and/or subsequent 
> balloon movement inside the guest?

The event is currently tracking when qemu events occur.

> Or, is my old version of qemu-kvm not passing this event back to 
> libvirt?


>  The latter doesn't seem correct because I'm getting seemingly-valid 
> values from dommemstat - ie, if I setmem 200MB when the guest OS and 
> applications are consuming 300MB, it reports near 300MB (and the guest 
> no longer responds) instead of just parroting the 200MB value I fed 
> it.

You are seeing different values because libvirt is doing polling under the
hood, when talking to a qemu too old to send the event.  If you upgrade qemu
to something that sends the JSON event, then libvirt will quit polling, and
instead rely on the event and forward the event back to you.

We could argue that since libvirt is already polling older qemu, that
libvirt should then synthesize VIR_DOMAIN_EVENT_ID_BALLOON_CHANGE any time
it happens to detect a change on one of its polls, but I'm not sure if it is
easier to write that patch than it is to just tell people to upgrade to
newer qemu.  Besides, with libvirt polling, the events will only happen
insofar as libvirt detects change, which might be at a different frequency
than how newer qemu would fire the events (note that the qemu 1.2
implementation rate-limits the event, to fire at most once per second, with
the last event firing as late as 1 second after the actual final change of
the memory).  That is, synthesizing the event in libvirt for older qemu
would require implementing the same rate-limiting code in libvirt.

Since you mentioned are using CentOS, you are at the mercy of what others
happen to package; if this feature is really important to you, you might
want to consider contributing patches upstream, and/or switching to a RHEL
contract to get Red Hat to implement it faster on your behalf.

Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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