Re: [Xen-users] Memory overcommitment in Xen.

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Jason Long <hack3rcon@xxxxxxxxx> wrote:

> But I like to know it did by Xen automatically or need to enable it?

You have to configure it.
In your guest VM configs, you need to specify both the maximum memory the guest 
is to have, and a setting for it's ballon driver to ballon down to after the 
guest has started - unless you are going to manually manage it.
Under older versions, these settings would do the job :
memory  = '2048'
maxmem  = '4096'
I vaguely recall some discussion here that it doesn't work under newer versions 
- but checking the most up to date host I have, it seems to be running that VM 
with 2G of memory.

But as already mentioned, you need to fully understand what's going on. The 
settings above will NOT start the VM with only 2G allocated. It will start the 
VM with 4G allocated and it relies on the balloon driver to "release" the spare 
2G. Depending on how you start the VMs, and what they do with memory while 
booting, this can have some undesirable performance issues. If you parallel 
start all the VMs, and they do things like scrubbing all the ram on boot, then 
you can cause some significant performance issues - in the very extreme, 
causing a guest to be killed.

https://blog.xenproject.org/2014/02/14/ballooning-rebooting-and-the-feature-youve-never-heard-of/


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