Re: [Xen-users] Dom0 Memory?

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 11 May 2012, Stefano Stabellini wrote:
> > On Fri, May 11, 2012 at 3:27 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
> > wrote:
> > > On Fri, 2012-05-11 at 09:11 +0100, Ian Tobin wrote:
> > 
> > >> Xl info
> > >>
> > >> total_memory      : 15869
> > >> free_memory      Â: 13124
> > 
> > 
> > >> Now I create a new config file for VM2 and I set the RAM
> > >>
> > >> memory = 12288
> > 
> > >> libxl: error: libxl.c:2150:libxl_set_memory_target new target 0 for dom0 
> > >> is
> > below the minimum threshold
> > >>
> > >> failed to free memory for the domain
> > >>
> > >> Why it is trying to take memory from dom0??
> > >
> > > You need to set "autoballoon=0" in /etc/xen/xl.conf if you don't want xl
> > > to automatically balloon dom0 down when starting a domain -- for example
> > > if you use the dom0_mem option.
> > >
> > > Ian.
> > 
> > Isn't that a bug? Shouldn't xl try to use free_memory first, and only
> > try to baloon-down memory from dom0 if it still needs more?
> 
> Even if this is an unsupported configuration, it could actually be a
> bug.
> Could you please post the output of xenstore-ls?

I didn't notice that you are actually trying to create a 12G VM.

The problem is that autoballoon=1 in xl.conf is fundamentally
incompatible with dom0_mem on the Xen command line.

The reason is that autoballoon calculates the amount of memory that
should be left free on the system but measuring how much memory is free
after dom0 boots, assuming that dom0 gets all the memory.
So if you pass dom0_mem=512M, then at boot time the free memory is
16G-512M => 15.5G.
Not so long ago xl autoballoon would have tried to keep 15.5G of memory
free in the system at all times, therefore preventing you from starting
any guests at all.
Fortunately we have a safety net for wrong configurations like this one:
the amount of memory that should be left free in the system is capped to
15% of the total memory, in this case 0.15*16G => 2.4G.

So the reason why you cannot start a 12G guest is that 12G > 13124M - 2.4G.

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