Re: [Xen-users] OS kernels ports, VT, Pacifica & performance

[Mogens Valentin <mogensv@xxxxxxxxxxxxxxxx>]

Sylvain Coutant wrote:
> Hi all,
>
> First, I'm not sure this post should have been sent to xen-devel or here. 
> Please, any list owner : forward to xen-devel if you read this and feel it 
> should have gone there.
>
> I wonder what will be the advantage, in terms of performance, of having 
> optimized kernels for XenU when VT/Pacifica will be there.
>
> AFAIK, using "standard" kernels means emulating peripherals (network card and so on) on dom0. Xen 
> "optimized" or "ported" kernels should have a performance advantage. But has this perf 
> increase already been evaluated ?
>
> Question behind this : does it worth the work to port some other OSes to Xen 
> architecture ?
Not having access to Pacifica/VT hardware (I'd almost kill for it), 
there are limits to what can be commented.
Think of normal multitasking. Handing over the cpu timeslice from one 
running process to another means saving the running process' state and 
loading another sleeping process' state.
With virtualization, the whole OS state and pagetable structure needs to 
be saved and revoked. A bit more timeconsuming..
Having virtualisation hardware support for this will be a real speed-up.
Having hardware support in a cpu with onchip memorycontroller will mean 
even more speed-up.
Without that onchip memorycontroller, quite a lot still needs be done in 
software. Yes, the cpu 'hardware' virtualization is not just a mix of 
registers and logic, but also firmware; however, still rather faster 
than what the target systems OS + virtualization mechanism can achieve.
With current virtualization techniques, guest/domU systems are always 
emulated, so having cpu hardware virtualization doesn't really change 
that, AFAICS. It'll mean the abilility to run unpatched OS's, though.
It's another ballgame further into the future, when the whole platform 
and PCIe gets increasingly virtualized. Maybe sometime around 2008..
--
Kind regards,
Mogens Valentin


PCIe virtualisation: Imagine cat herding with a firehose
and firecrackers.  That is notably easier than getting all
the peripheral makers to play along.
  -- fun on theinquirer.net


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