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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Fri, Nov 23, 2012 at 12:47:07PM +0000, Ian Campbell wrote:
> On Fri, 2012-11-23 at 12:41 +0000, Pasi Kärkkäinen wrote:
> > On Fri, Nov 23, 2012 at 12:33:59PM +0000, Ian Campbell wrote:
> > > On Fri, 2012-11-23 at 12:28 +0000, Pasi Kärkkäinen wrote:
> > > > On Fri, Nov 23, 2012 at 12:21:32PM +0000, Ian Campbell wrote:
> > > > > This question belongs on xen-users.
> > > > > 
> > > > > On Fri, 2012-11-23 at 11:35 +0000, Alexander Bienzeisler wrote:
> > > > > > Hello,
> > > > > > 
> > > > > > i just tried to add a domU with 16 gigs of ram (maxmem = 16384, 
> > > > > > memory = 
> > > > > > 16384) with free_memory : 28969 but got an error:
> > > > > > 
> > > > > > libxl: error: libxl.c:3433:libxl_set_memory_target: new target 0 
> > > > > > for 
> > > > > > dom0 is below the minimum threshold
> > > > > 
> > > > > The clue is in this message -- you cannot safely balloon down a guest
> > > > > (including dom0) to an arbitrarily small value relative to its initial
> > > > > size. The kernel will have certain fixed datastructures etc which are
> > > > > sized according to the maximum amount of RAM at start of day.
> > > > > 
> > > > > If you were to balloon down too low then you would run out of RAM and
> > > > > the OOM killer would step in and start killing stuff, so xl includes a
> > > > > safety catch which prevents this.
> > > > > 
> > > > > If you have a huge host and want a small dom0 use the dom0_mem option
> > > > > and disable xl's autoballoon option or whatever the equivalent xend
> > > > > option is.
> > > > > 
> > > > 
> > > > I think he was already using dom0_mem. He had 28969 free in Xen as
> > > > reported by "xl info".
> > > 
> > > Then he's missed disabling autoballoon.
> > > 
> > 
> > As can be seen from the other email in this thread,
> > he's using dom0_mem=2048M, and he has 28GB of free memory in Xen.
> > 
> > Why is xl trying to balloon down dom0 (and failing) for no reason? 
> 
> Because autoballoon is enabled.
> 

Yep, autoballoon=0 fixed the problem.

> 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..

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.. 
not-yet-implemented check/feature in xl, or a bug? 

Thanks,

-- Pasi


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