Re: [Xen-devel] Re: VT-X processors , xen 3.0 , drives and virtualization (in 32 bit environment)

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Ian Brown wrote:

> Hello,
>  Ok, Thnks,this clears a lot about the first question.
>
> Still, I will be grateful if I get any feedback on the second question
> : performance overhead of running more than one Linux OS instance on
> these VT-processors :
> did anybody tried it and can comment on it / give some data / compare to
> non VT processors
> (which have aboutb 3% performance overhead) ? I would expect that
> somebody had tried using these
> VT chips (even they are still (maybe) in beta stage ).
>  
VT support in Xen 3.0 is still mostly proof-of-concept.  It's not really 
fair to use it to analyze the performance of VT since there hasn't been 
a lot (although there has been some) performance work done on VT.
The Xen VT support takes a pretty bad performance hit because of the 
fact that the device model is run in a different domain so there's a 
pretty big hit from world/context switches.
Stay tuned though, because there's no reason why future versions of Xen 
shouldn't perform quite well under VT :-)
Regards,

Anthony Liguori

> Regards,
> IB
>
>
> On 12/27/05, Charles Duffy <cduffy@xxxxxxxxxxx> wrote:
>  
>
> > Ian Brown wrote:
> >    
> >
> > > 1) True to now and the current xen-3.0 version: When running Xen 3.0
> > > on these VT processors, can we run an unmodified kernel ?
> > >      
> > For your DomUs, yes. For your Dom0, no. Running a modified DomU kernel
> > should be more performant.
> >
> >    
> >
> > > and in such a case, what about the device drivers - isn't there a
> > > problem with them ?
> > >      
> > Emulated hardware, based off of drivers borrowed from QEMU, is provided
> > to VMX domains. The drivers they need will be for the emulated cards,
> > not for the real devices in the machine.
> >
> >    
> >
> > > I mean , in practical terms , if I will set my bootloader to have the
> > > following entry on a machine with VT-x processor:
> > > kernel /xen.gz dom0_mem=x
> > > module /vmlinuz-#version ro root=...
> > > (and initrd if needed)
> > >      
> > No, that won't work, because that's trying to use an unmodified kernel
> > as Dom0.
> >
> >    
> >
> > > will I be
> > > able to create a new domain based also on unmodified vmlinuz-#version
> > > kernel ?
> > >      
> > Yes, though VMX domains work a bit differently from non-VMX ones -- the
> > process won't be exactly the same except with a non-Xen-enabled kernel.
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >
> >    
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>
>  

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