Re: [Xen-devel] can not use all available memory

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Fri, Nov 23, 2012 at 01:15:53PM +0000, Ian Campbell wrote:
> On Fri, 2012-11-23 at 12:55 +0000, Pasi Kärkkäinen wrote:
> 
> > > This has been discussed at length on the list before, please check the
> > > archives.
> > > 
> > 
> > I tried googling quickly but I didn't find anything relevant..
> 
> I found
> http://web.archiveorange.com/archive/v/zCKz5T3PLvtyZDSPQc9i
> in a matter of seconds, then:
> http://lists.xen.org/archives/html/xen-devel/2011-11/msg01415.html
> http://lists.xen.org/archives/html/xen-users/2012-05/msg00139.html
> http://lists.xen.org/archives/html/xen-users/2012-05/msg00146.html
> 
> > To me this behaviour still seems wrong. What's the point of autoballoon=1 
> > trying to 
> > balloon down dom0 if the hypervisor already has enough free memory for the 
> > VM ? 
> > 
> > In this case:
> >     - dom0_mem=2G
> >     - new VM to launch with size 16 GB.
> >     - Xen has 28 GB of free memory.
> > 
> > So clearly there's no need to try to balloon down dom0.. 
> 
> Right, so don't set autoballoon then.
> 

It's enabled as a default.. so many people hit this problem.


> > not-yet-implemented check/feature in xl, or a bug? 
> 
> Neither, it is the intended behaviour of xl autoballoon, this option
> means exactly "take the required memory from dom0".
> 

http://xenbits.xen.org/docs/4.2-testing/man/xl.conf.5.html

"autoballoon=BOOLEAN

    If disabled then xl will not attempt to reduce the amount of memory 
assigned to domain 0 in order to create free memory when starting a new domain. 
You should set this if you use the dom0_mem hypervisor command line to reduce 
the amount of memory given to domain 0 by default.

    Default: 1"


I think we should modify that to say "You should set autoballoon=0 if you use 
the dom0_mem hypervisor command line .." 
At least I understood that text in the opposite way..

Also: What happens if you have autoballoon=1 and you start some VMs, then stop 
the VMs, 
so most of the memory is now free in Xen.. and then you try to start a big VM ? 
Aren't you going to hit the same problem as in this thread? 


-- Pasi


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