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RE: [Xen-users] how much memory does the hypervisor need

To: "Philipp Schmid" <lists@xxxxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] how much memory does the hypervisor need
From: "Gary W. Smith" <gary@xxxxxxxxxxxxxxx>
Date: Tue, 17 Feb 2009 13:48:51 -0800
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Thread-topic: [Xen-users] how much memory does the hypervisor need
I've been pre-allocating 384mb (by setting it in grub) on all of my xen hosts (running CentOS 5.2 / xen 3.3.0 -- gitco).  I started doing this some time ago because I run into baloon driver issues when I don't do it (where some or all of the DomU's will lose network connectivity).  This value seems to work well for us.  Then again, we are running very base Dom0's.  That is, minimal install + xen. 
             total       used       free     shared    buffers     cached
Mem:        393216     333288      59928          0       8376     160560
-/+ buffers/cache:     164352     228864
Swap:      8385920        172    8385748
I'm not sure what the best practive is though.  What I do know is that by doing it this way when you run xm top you will know exactly how much memory is available to allocate to new VM's.
PS: sorry for top posting, OWA doesn't give me the option.

From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx on behalf of Philipp Schmid
Sent: Tue 2/17/2009 1:29 PM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] how much memory does the hypervisor need


I wonder how much memory the hypervisor needs. I have the following 
vms running:

xen26:~# xm list
Name                                        ID   Mem VCPUs      
State   Time(s)
Domain-0                                     0   725     1     
r-----   2064.7
g-hlsan9ib                                   1  4096     4     -
b----    160.9
g-lp549ugs                                  19  1024     4     -
b----     83.0
g-lpjj78e9                                   2  1024     2     -
b----    156.1
g-nt731fct                                  17  2048     2     -
b----    116.5
g-resl3iqi                                   4  2048     4     -
b----    326.7
g-rujbc3il                                   5  2048     4     -
b----    192.6
g-v72lkt0b                                   6  1024     2     -
b----    197.5

725+4096+1024+1024+2048+2048+2048+1024 = 14037mb

dom0 has fixed 725mb of memory, the server has 16gb of memory.

if try to launch another 2048mb vm i get the following:

xen26:~# xm create /etc/xen/g-ida7k5fo.cfg
Using config file "/etc/xen/g-ida7k5fo.cfg".
Error: Not enough free memory and enable-dom0-ballooning is False, so 
I cannot release any more.  I need 2138112 KiB but only have 1930744.

so if i take the 16384mb of ram minus the used space and the space xen 
sys it still has i get:

16384 - 14037 - 1930744/1024 ~= 461.5mb

does the hypervisor really need 460mb of memory?
is there a way to calculate how much memory the hypervisior needs?

lg philipp

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