[Gllug] performance of xen dom0 vs native linux

[Richard Jones]

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.

-- 
Richard Jones
Red Hat
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