[CentOS-virt] centos6 and xen4 dom0 memory allocation

Peter Peltonen peter.peltonen at gmail.com
Sun Oct 23 13:54:13 EDT 2011

Replying to myself:

On Sun, Oct 23, 2011 at 5:51 PM, Peter Peltonen
<peter.peltonen at gmail.com> wrote:
> There is something fishy going around... I will now try to yum update
> dom0 and domUs, set the dom0-min-mem to 2048M and reboot and see what
> happens.

After rebooting I seemed to have 2048M in dom0 regardless of how many
domUs were running.

But when trying to create the last domU, I was informed that xend was
out of memory. At this time I still had something about 5G of real
memory available for the  last domU which I have configured to consume
4G of memory.

I tweaked grub.conf and xend-config.sxp and set 4096M as the dom0
memory and rebooted again. I reduced the domU memory sizes accordingly
and all domUs got created without errors. But the strange thing is
again dom0 memory which has been reduced from 4096M to 2052M:

[root at si ~]# cat /etc/xen/xend-config.sxp |grep dom0-min-mem
# dom0-min-mem is the lowest permissible memory level (in MB) for dom0.
(dom0-min-mem 4096)

[root at si ~]# xl info |grep mem
total_memory           : 24567
free_memory            : 5853
xen_commandline        : dom0_mem=4096M dom0_max_vcpus=1 dom0_vcpus_pin

[root at si ~]# xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0  2052     1     r-----     265.6

xl top reports "(MEM)k" as "2101272" and "MAXMEM(k)" as "no limit".

In CentOS5 with Xen3 I have been used to having the amount of memory
in dom0 which I have specified. My Xen4 installation seems not to do

If someone else is using the Xen4 packages from
http://xenbits.xen.org/people/mayoung/EL6.xen I would be interested in
hearing about your experiences?


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