[Fedora-xen] network speed on hvm guests

[John Summerfield]

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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