On Wed, Jul 17, 2013 at 5:42 PM, David Vrabel <david.vrabel@citrix.com> wrote:

On 16/07/13 14:18, Rushikesh Jadhav wrote:
>
>
>
> On Tue, Jul 16, 2013 at 3:26 PM, David Vrabel <david.vrabel@citrix.com

> <mailto:david.vrabel@citrix.com>> wrote:
>
> On 15/07/13 20:44, Rushikesh Jadhav wrote:
> > Hi List,
> >
> > It seems that the xen balloon functionality for guest is not
> enabled in
> > CentOS 3.4.50-8.el6 xen kernel by default via
> centos-release-xen.x86_64
> > repo. I think this should have been present since this extra Xen repo
> > needs to support as both Guest and Host (Dom0).
>
> The kernel is only intended for use as a dom0 kernel. We recommend the
> use of the existing 2.6.32 kernel for guests.
>
>
> Any idea for not using 3.4.50 as guest kernel ?

The specific 3.4 kernel provided as part of the Xen4CentOS project as a
configuration selected for use as a dom0 kernel and thus it may not have
guest-centric options (like ballooning) correctly configured.

> Problem may be with CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=y as well. What do
> you think ?

CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=y is required to allow most Linux PV
guests to balloon above their initial allocation. However, it is broken
in some cases when enabled in a dom0 kernel and has been disabled.

Can we enable it with a code check for xen_initial_domain() ?

We really need an authenticated guest Centos kernel which has bug free ballooning as Xen guest.

KVM or HVM Xen is fine but PV will always suffer till CentOS/RHEL switches to 3.X

I don't have any news on CentOS7 or RHEL7.

As a workaround I would suggest starting all guests with an initial
allocation that corresponds to the maximum they may ever use. You can
then immediately balloon them down.

This workaround works good only while first boot. If the guest gets rebooted after balloon down then the base memory is changed to last known allocated memory. After reboot the guest wont be able to balloon up.

David