Re: [Xen-users] xen in xen in xen....

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-02-28 at 09:09 +0000, Gordan Bobic wrote:
> My findings a while back were that at 1st level you should expect a 20% 
> performance drop (Note: Xen beats the competition here - other hypervisors do 
> worse.).
> 
> More details here:
> http://www.altechnative.net/2012/08/04/virtual-performance-part-1-vmware/
> 
> I am not sure what the overhead scaling is when nesting - it could be
> close to linear (multiply by 0.8 for each layer) or it could be worse
> due to greater cache contention and increased context switching.

My gut reaction is that it would be worse, due to the "towers of hanoi"
affect. i.e. if you have L0 -> L1 -> L2 hypervisors then a trap in L2
will trap to L0, which will decide it is destined for L1 and so inject
it, then L1 will do whatever is needed to satisfy the trap, which itself
is likely to trap to L0 which will do whatever it needs, etc etc and you
end up bouncing up and down throughout the stack lots of times to
satisfy a single trap in the deepest hypervisor.

Ian.



> 
> 
> Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> 
> >On Thu, 2014-02-27 at 14:26 -0600, John Wargo wrote:
> >> I don't have any experience with it, but looking at the configuration
> >> below it looks possible.
> >> http://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html
> >> (Search for "nestedhvm=BOOLEAN")
> >
> >In principal you should be able to nest as far as you like but a) I
> >expect almost no one else has tried this and b) once you go more than a
> >single level of nesting performance is going to tail off pretty heavily.
> >
> >Ian.
> >
> >
> >
> >
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