Xen users - Jan 2006 - VT or paravirtualization?

Hello,

What is the better solution now to run linux domUs under Xen3?
Using a VT hardware, sould I install a xen-patched kernel as I would
where no VT extension, or use an unmodified linux guest system?

Is there any performance difference between a Pentium D 3 Ghz with vt
(9xx) and without it (8xx)? 

Regards,
Slapic



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> What is the better solution now to run linux domUs under Xen3?
> Using a VT hardware, sould I install a xen-patched kernel as I would
> where no VT extension, or use an unmodified linux guest system?
If you can run a kernel that''s been ported to Xen it''ll be
faster (and more
stable, at this point, since that code is more mature than the VT code) than 
using an unpatched kernel.  You also get features like suspend/resume and 
migration, which are not currently available for unpatched guests.
> Is there any performance difference between a Pentium D 3 Ghz with vt
> (9xx) and without it (8xx)?
No idea... are the specs different in any other way?  Remember VT isn''t
used
*at all* when running a Xen-patched kernel, so having VT features enabled 
will not speed up the performance of that kind of guest.  If you don''t
want
unmodified guests, then there''s no need to get VT.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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> -----Original Message-----
> From: xen-users-bounces@lists.xensource.com 
> [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of 
> Czakó Krisztián
> Sent: 29 January 2006 21:03
> To: xen-users@lists.xensource.com
> Subject: [Xen-users] VT or paravirtualization?
> 
> Hello,
> 
> What is the better solution now to run linux domUs under Xen3?
> Using a VT hardware, sould I install a xen-patched kernel as 
> I would where no VT extension, or use an unmodified linux 
> guest system?
At this time, I''d say that Para-virtualization is better performing.
This is based on the fact that we''re currently on the first release of
Hardware virtualization, and there''s several things that are less than
optimal. Device model optimisation for specific devices would be one clear
benefit, but there''s other possibilities too.

Obviously, if you want to run unmodified OS''s that doesn''t
have freely available source, or perhaps run an application that REQUIRES a
particular distribution that can''t have a modified kernel, then full
virtualization is pretty much the only option.
> 
> Is there any performance difference between a Pentium D 3 Ghz with vt
> (9xx) and without it (8xx)? 
I can''t speak for Intel, but from AMD''s perspective [1] I
would expect there to be NO difference for your average application software -
kernel mode instructions such as control register access and such things MAY be
a tiny bit slower (but these are generally VERY EXPENSIVE instructions ANYWAYS,
for example a move to CR3 will invalidate the TLB''s on the processor,
which means that the very next memory read will require reading of all levels of
the page-table [ignoring the fact that there''s optimisations in the
processor that "ignore" moves to CR3 with the same value]).

I''d doubt that any REAL application benchmark would be able to show any
difference that is statistically verifiable.

[1] I''m not a processor architect, so this is MY _OPINION_, rather than
a statement of facts from AMD as a corporation.

--
Mats
> 
> Regards,
> Slapic
> 
> 
> 
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