Re: [Xen-users] Re: Why pv-on-hvm drivers?

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Sep 06, 2010 at 12:55:05PM +0300, Pasi Kärkkäinen wrote:
> On Mon, Sep 06, 2010 at 10:59:26AM +0200, Markus Schuster wrote:
> > Pasi Kärkkäinen wrote:
> > > On Sun, Sep 05, 2010 at 01:29:19AM +0200, Markus Schuster wrote:
> > >> Hi list,
> > >> 
> > >> I've read about recent efforts to push pv-on-hvm drivers to Linux
> > >> mainline and I'm curious to know the cause for this. What's the advantage
> > >> over using pv_ops directly and booting the kernel paravirtualized?
> > > 
> > > The other point is performance: 32bit PV (paravirtualized) guests
> > > perform OK, but 64bit PV guests have a performance hit if your
> > > workload creates a lot of new processes in the guest.
> > > 
> > > HVM helps there; 64bit Linux guests might be faster as HVM,
> > > depending on the workload.
> > 
> > Hi Parsi, thanks for your (as usual :)) good answer. 
> > That's the first time I read about a PV performance hit compared to HVM - 
> > maybe you (or someone else) can write a few words about what's causing 
> > that? 
> > Could be interesting for other people, maybe?
> > 
> 
> I think there are some XenSummit presentations about it on xen.org website.
> It has to do with 32bit vs 64bit architecture differences related to memory 
> management.
> 
> Every time a new process is created by the 64bit PV kernel
> the guest process pagetables need to be verified/checked by the hypervisor,
> and this causes a performance hit if you need to create a lof 
> of new processes in the guest.
> 
> It doesn't affect 'long running' processes in a 64bit PV guest,
> ie. the performance hit happens only when new processes 
> are created often (kernel compilation, unixbench).
> 
> For an HVM guest that stuff is handled by the CPU/hardware,
> so there's no performance hit related to it. 
> HVM guests have some other performance hits though..
> 
> That's my understanding of it :)
> 

I think this video interview of Keir Fraser includes that stuff:
http://blog.xen.org/index.php/2009/07/02/developer-interview-series/

>From around 8 minutes on that video..

-- Pasi


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