[Gllug] performance of xen dom0 vs native linux

[Nahuel Marisi]

2009/5/8 Richard Jones <rich at annexia.org>

> On Fri, May 08, 2009 at 10:41:42AM +0100, Nahuel Marisi wrote:
> > Hi there,
> > I'm currently using xen for various experiments (as it's much easier and
> > cheaper than running two or three linux boxes).
> > As I tend to use Xen frequently I usually boot directly into it even when
> I
> > don't actually fire up any VMs.
>
> Please ignore the other reply by "Juergen Schinker" - it's uninformed
> nonsense.
>
> > I was wondering what's the difference in performance between linux
> running
> > in dom0 and no VMs (domUs) running compared to native linux.
>
> There's a small performance difference.  Dom0 under Linux will be a
> few percentage points slower.  You'll not be likely to notice it
> unless you're doing some very particular things.
>
> > Obviously if you use something like virtualbox when there are no VMs
> running
> > you just have linux running natively so you have no performance
> penalties.
> > Is it the same if you're running Xen with no hosts?
>
> No ...  You always have the small penalty of the hypervisor.
>
> > Does it affect the way linux runs in any way?
>
> Yes.  Dom0 is a 'special' guest.  It's a guest running a
> paravirtualized kernel.  But it also has direct access to most
> hardware, which other (DomU) guests won't have.
>
> One implication of this are that there is slightly less physical RAM
> available.  Also any resource which is intermediated by the hypervisor
> -- memory, CPU flags, interrupts -- can be problematic because you're
> relying on the Xen hypervisor to support your hardware fully in each
> of these areas.
>
> A more serious problem is with features like powersaving and suspend.
> The hypervisor doesn't know how to powersave or suspend, and the dom0
> can't make the right decisions because it doesn't know what's
> happening in the other domains.  This can make Xen less than useful on
> laptops.
>
> Then there is the general mess of Xen network scripts.
>
> This is of course why it's better to use Linux _as_ the hypervisor.
> Hence KVM ...
>
> Rich.



Thanks for the detailed reply Rich.
I haven't actually played around with KVM yet (it's next on the list).
However, isn't KVM slower than Xen since it has to emulate
most of the hardware (ie, hardrive controller, graphics card, etc) through
the use of qemu? As oppose to Xen which has direct access to it, and uses
dom0 kernel modules?

Unfortunately it is running on a laptop and I actually have experience
problems with fans running more than usual (although it's hard to say if
it's Xen's fault).
If it uses the dom0's kernel modules can't it Xen (who is aware of all
virtual guests) make good decisions about power saving?

Also KVM requires hardware virtualization. Isn't that slower than
paravirtualization ?


>
>
> --
> Richard Jones
> Red Hat
> --
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