Re: [Xen-devel] Re: [PATCH] xen: core dom0 support

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

George Dunlap wrote:
> On Wed, Mar 4, 2009 at 5:34 PM, Anthony Liguori <anthony@xxxxxxxxxxxxx> wrote:
>   
> > Can you point to benchmarks?  I have a hard time believing this.
> >
> > How can shadow paging beat nested paging assuming the presence of large
> > pages?
> >     
> If these benchmarks would help this discussion, we can certainly run
> some.  As of last Fall, even with superpage support, certain workloads
> perform significantly less well with HAP (hardware-assisted paging)
> than with shadow pagetables.  Examples are specjbb, which does almost
> no pagetable updates, but totally thrashes the TLB.
I suspected specjbb was the benchmark.   specjbb is really an anomaly as 
it's really the only benchmark where even a naive shadow paging 
implementation performs very close to native.
specjbb also turns into a pathological case with HAP.  In my 
measurements, HAP with 4k pages was close to 70% of native for specjbb.  
Once you enable large pages though, you get pretty close to native.  
IIRC, around 95%.  I suspect that over time as the caching algorithms 
improve, this will approach 100% of native.
Then again, there are workloads like kernbench that are pathological for 
shadow paging in a much more dramatic way.  At least on shadow2, I was 
seeing around 60% of native with kernbench.  With direct paging, it goes 
to about 85% of native.  With NPT and large pages, it's almost 100% of 
native.
>   SysMark also
> performed much better with shadow pagetables than HAP.  And of course,
> 64-bit is worse than 32-bit.  (It's actually a bit annoying from a
> default-policy perspective, since about half of our workloads perform
> better with HAP (up to 30% better) and half of them perform worse (up
> to 30% worse)).
>
> Our comparison would, of course, be comparing Xen+HAP to Xen+Shadow,
> which isn't necessarily comparable to KVM+HAP.
>
> Having HAP work well would be great for us as well as KVM.  But
> there's still the argument about hardware support: Xen can run
> paravirtualized VMs on hardware with no HVM support, and can run fully
> virtualized domains very well on hardware that has HVM support but not
> HAP support.
>   
Xen is definitely not going away and as such, supporting it in Linux 
seems like a good idea to me.  I'm just refuting claims that the Xen 
architecture has intrinsic advantages wrt MMU virtualization.  It's 
simply not the case :-)
Regards,

Anthony Liguori

>  -George Dunlap
>   

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