Fedora xen - Jan 2008 - Re: network speed on hvm guests

On Fri, Jan 04, 2008 at 11:57:09AM -0500, Dustin Henning
wrote:
> 	I was reading an article about Windows PE 2.0 in virtual
> environments, and apparently Windows PE 2.0 is based on Vista, which
doesn''t
> have a driver for the default NIC used by VMWare.  The suggested solution
is
> to use a different virtual NIC (a gigabit one).  Obviously Xen
isn''t VMWare,
> and on top of that, I am not trying to run Windows PE 2.0, or even Vista.
> However, I had long wondered but never asked this question:
> 	Does Xen have any virtual NICs other than the two mentioned in the
> hvm example files in /etc/xen?  More importantly, does it matter, for
> instance, will Xen or Windows limit the response time or throughput to
> 100MB/s levels in an hvm environment if a 100MB/s virtual NIC is used?  I
> don''t see why it would, but then again, I don''t see why
SATAII hard drives
> running on controllers in IDE mode are slowed to IDE speeds, so I
wouldn''t
> be surprised if it did, and I am hoping someone can tell me.  Thanks,
> 	Dustin
> 
Xen HVM guests see _emulated_ hardware. Emulated NIC, Emulated ide/disk
controller, etc.

Emulation is slow. 

You need to have/install paravirtualized (=optimized) drivers that
"bypass"
the whole emulation layer and talk directly to Xen.. this way you can get
good performance. 

Xen includes paravirtualized drivers ("unmodified drivers") for linux
HVM
guests.

People are working with free/opensource paravirtualized drivers for Windows
HVM guests atm, but they''re not really usable/stable yet.

XenEnterprise, VirtualIron, and Novell SLES have Windows HVM pv drivers
available.. if you buy those products. 

-- Pasi

Pasi � wrote:
> On Fri, Jan 04, 2008 at 11:57:09AM -0500, Dustin Henning wrote:
>> 	I was reading an article about Windows PE 2.0 in virtual
>> environments, and apparently Windows PE 2.0 is based on Vista, which
doesn''t
>> have a driver for the default NIC used by VMWare.  The suggested
solution is
>> to use a different virtual NIC (a gigabit one).  Obviously Xen
isn''t VMWare,
>> and on top of that, I am not trying to run Windows PE 2.0, or even
Vista.
>> However, I had long wondered but never asked this question:
>> 	Does Xen have any virtual NICs other than the two mentioned in the
>> hvm example files in /etc/xen?  More importantly, does it matter, for
>> instance, will Xen or Windows limit the response time or throughput to
>> 100MB/s levels in an hvm environment if a 100MB/s virtual NIC is used? 
I
>> don''t see why it would, but then again, I don''t see
why SATAII hard drives
>> running on controllers in IDE mode are slowed to IDE speeds, so I
wouldn''t
>> be surprised if it did, and I am hoping someone can tell me.  Thanks,
>> 	Dustin
>>
> 
> Xen HVM guests see _emulated_ hardware. Emulated NIC, Emulated ide/disk
> controller, etc.
> 
> Emulation is slow. 
Compared with hardware, yes. The bit that emulates hardware, yes. Expect 
the Windows driver to run full speed, it''s when there''s an
interrupt to
switch to the emulator that the rot sets in.

I would expect any emulated network device to go as fast as the hardware 
permits, and not to be constrained to any notion of the emulated 
device''s speed. A real device is constrained by its need to talk to 
other hardware (electronic signal timing and such) whereas an emulated 
device is talking to more software.

A more complex device will be harder to emulate and so run slower.

-- 

Cheers
John

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