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

["Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>]

On 2016-12-05 03:01, Simon Hobson wrote:
> 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.
I think you're thinking of a thread I had been involved with which 
discussed transcendent memory and overcommit, and starting pre-ballooned 
does not currently work correctly with Linux guests when using 
transcendent memory.  It does however work right without having tmem 
enabled in the guest OS.
> 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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