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

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Date:	Thu, 5 Mar 2009 10:59:58 +0000
From:	George Dunlap <George.Dunlap@...citrix.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
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

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.  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.

 -George Dunlap
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