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RE: [Xen-users] Config: Paravirtualization and Full Virtualization

To: "MJang" <mike@xxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] Config: Paravirtualization and Full Virtualization
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Thu, 1 Feb 2007 16:14:14 +0100
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Thread-topic: [Xen-users] Config: Paravirtualization and Full Virtualization

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of MJang
> Sent: 01 February 2007 14:52
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-users] Config: Paravirtualization and Full 
> Virtualization
> On Wed, 2007-01-31 at 11:54 +0200, Graeme Gerber wrote:
> > Where's the difference in the guest installation (or maybe 
> in the host
> > installation?) when you install a Linux in 
> paravirtualization mode, or
> > in full virtualization mode (VT or pacifica)?
> > 
> > g>> Full gives better performance from what I hear.  Your hardware
> > should be suitable and the bios option enabled.
> > If you know anything about bios pls do let me know as Sony have
> > disabled this option in there bios.
> Full gives better performance for the Xen client - but since Para does
> not require complete hardware emulation (and requires optimized Xen
> kernels), it results in better performance overall, especially if you
> have multiple Xen clients.

Even if you don't touch any IO hardware, I don't think hardware
virtualization is noticably faster (if faster at all) than
Para-virtualization (and I have a good reason to NOT say this, but I say
it, because I believe it's the case, currently at least). Of course if
the guest is doing absolutely nothing that the hypervisor needs to know
about, there's very little difference in the two cases, as it's 99.9%
about the actual speed of the system itself (CPU and memory, as other
components, such as disk and network, are controlled via the hypervisor
in one way or another). But assuming we're running something that
doesn't do disk-access or network-access, but needs a bit of help from
the hypervisor for other aspects, such as memory management, I would say
that para-virtual is either going to be faster or same speed for the
same task. 

One thing that will change this is the ability to use "Nested paging" -
that will allow the hypervisor to give the VM it's own memory region,
mapping for example 0..256MB of "guest memory" to a section of "machine
memory" that is 256MB somewhere in the machine. By this extension to the
architecture (which is already in the AMD specs), it's possible for the
guest to run almost autonomously with a very small overhead. A hybrid of
this technique and para-virtualization is also technically possible,
where a very thin/small hardware virtualization layer is used in
conjunction with an otherwise para-virtual OS - that way achieving the
best of both worlds. 

There are other reasons to use full virtualization today, and one of
those is the inability to xenify all available operating systems, either
due to lack of available source code or lack of resources.


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