[CentOS-virt] XEN and RH 6

Pasi Kärkkäinen pasik at iki.fi
Tue Nov 10 21:36:39 UTC 2009

On Tue, Nov 10, 2009 at 01:52:36PM -0500, Scott McClanahan wrote:
> > Yeah.. Xen paravirtualized mmu is fast, and in some (many) cases beats
> > CPU hardware virtualized mmu.
> > 
> > KVM has 'pvmmu' aswell, but it's not as good, so KVM is faster with CPU
> > hardware virtualization. But that's a problem of KVM only, they haven't
> > managed to optimize the pvmmu. And they're going to drop it altogether.
> > 
> > KVM people tend to say 'paravirtualized mmu is slow', but they just mean
> > KVM implementation of it sucks :)
> > 
> > -- Pasi
> I haven't tested or seen any benchmarks but I wonder how much the
> addition of a page table for virtualized guests will help.  Not to
> mention newer features like a virtualized task priority register and
> ASID could continue to require less paravirt code in the guest.  I get
> my two new 5500 series servers in a few weeks so I'm pretty excited to
> see some of the second gen hardware virtualization assist features in
> action.

I don't know. Of course hardware will add features and get more
optimized in the future.

Some benchmarks from IBM guys, Xen vs. KVM:


"So, KVM requires 66.93/52.85 = 26.6% more CPU to do the same amount of work."
"If we normalize to CPU utilization, Xen is doing 20% more throughput."
"KVM running Windows VMs uses 46% more CPU than the Other-Hypervisor"
"A different hypervisor was compared; KVM used about 60% more CPU cycles to complete the same amount of work."

I bet KVM will catch up at some point.. at the moment it seems to not 
perform as good as Xen. Then again it's a much younger product.

-- Pasi

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