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

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Date:	Thu, 05 Mar 2009 08:37:04 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	George Dunlap <George.Dunlap@...citrix.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Xen-devel] Re: [PATCH] xen: core dom0 support

George Dunlap wrote:
> On Wed, Mar 4, 2009 at 5:34 PM, Anthony Liguori <anthony@...emonkey.ws> 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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