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Re: [Xen-users] dom0 memory usage >1GB at startup (no domU's configured)

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Subject: Re: [Xen-users] dom0 memory usage >1GB at startup (no domU's configured)
From: "Nick Couchman" <Nick.Couchman@xxxxxxxxx>
Date: Sun, 04 Apr 2010 08:32:32 -0600
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Use "top" and sort by memory usage to see what's taking up the memory.  Also, 
you might consider specifying "dom0_mem" on the xen kernel boot line, as this 
will limit the memory that dom0 sees, which will affect how memory is allocated 
on the system.

Don't be fooled by the "xm info" output, or the "xm top" output, for that 
matter.  No, this does not mean that only 2GB of RAM are available for guests.  
I'm assuming you've already started up one domU on that machine?  Xen 
automatically balloons memory on dom0 as guests (domUs) are started - so, if 
you start a guest that takes up 4GB, you'll see the dom0 memory decrease to 4GB 
and the rest will be allocated to the domU.  Xen does *not* automatically 
reassign memory back to dom0 after a domU is shut down - dom0s memory remains 
lower and the memory is marked as "free" to Xen.  When you first start up your 
machine, before you launch any domUs, you should see close to 0 MB listed as 
"free" in the "xm info" screen.  This is another reason why it's good to set 
dom0_mem at boot time - it'll give you a much better idea of how much memory 
you actually have available using the xm commands.


>>> stalks  04/03/10 8:44 PM >>>
Dear list,

I have a new install of Debian Squeeze.

I have followed the hints from the ParaVirtOps wiki page to grab 
Jeremy's kernel sources from kernel.org and compiled a 2.6.31.x (and 
2.6.33.x) kernel.

I have yet to install any significant software other than package 

The machine is a Core i7 920 with 8GB RAM.

If I boot the plain kernel (without xen-3.4.gz) I get ~65MB ram usage 
shown in free -m.

If I boot with full xen, bear in mind I have no domU's yet, I get the 

 > free -m
              total       used       free     shared    buffers     cached
Mem:          7948       1412       6535          0          9        107
-/+ buffers/cache:       1295       6652
Swap:            0          0          0

 > xm info
host                   : oracle
release                :
version                : #1 SMP Sun Apr 4 00:21:54 BST 2010
machine                : x86_64
nr_cpus                : 8
nr_nodes               : 1
cores_per_socket       : 4
threads_per_core       : 2
cpu_mhz                : 2660
hw_caps                : 
virt_caps              : hvm
total_memory           : 8183
free_memory            : 2174
node_to_cpu            : node0:0-7
node_to_memory         : node0:2174
xen_major              : 3
xen_minor              : 4
xen_extra              :
xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 
hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xffff800000000000
xen_changeset          : unavailable
cc_compiler            : gcc version 4.4.3 (Debian 4.4.3-2)
cc_compile_by          : waldi
cc_compile_domain      : debian.org
cc_compile_date        : Mon Mar  1 21:16:18 UTC 2010
xend_config_format     : 4

 > xm top
     xentop - 03:33:21   Xen 3.4
     1 domains: 1 running, 0 blocked, 0 paused, 0 crashed, 0 dying, 0 
     Mem: 8379516k total, 6152576k used, 2226940k free    CPUs: 8 @ 2660MHz

(free -m): Over 1GB used inside dom0 already?
(xm info): Only 2GB available for guests?
  (xm top): Over 6GB used?

The only processes running outside of bare minimum are xinetd, sendmail, 
mdadm and ssh.

I can stop xend and xendomains from starting, and I still have 1GB used 
at startup.

The same setup on a lesser spec'd machine is working fine with ~140Mb 
used at boot. I've spent a few hours on this and am at a loss.

Some info can be found at the following sources:


Someone show me the light!


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