Re: [Xen-devel] Possibilities with Xen3 compared to IBM Power5 DLPAR

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I'm just comparing the current development state of Xen with the
> established technology from IBM on current Power5 series.
> I could'nt find any information to these questions on the internet...

Are we talking about Xen/PPC or Xen/x86 here?

> 1. Does Xen support capacity entitlement for the CPUs, e.g.
>    for processor pooling? (does Xen classify the CPU capacity
>    into time slices? Would be the first step...)
>
> 2. If question 1 is "yes" or "will be in some day", is/will
>    there be support for capped and uncapped domU's so that you can
>    mix up things?
>
> 3. If uncapped use of a processor pool is possible, how about weighting
>    different domU's? (e.g. dom0 should have uncapped mode with the
>    highest weight)

I think the credit scheduler supports at least some of this.

> 4. Where can I find any further information about the dynamical
>    change of active VCPUs and memory settings? E.g. What the domU
>    guest system needs to have for supporting this.

VCPU hotplug Just Works for XenLinux guests, I think.  Memory resizing also 
works for XenLinux but it's worth noting that if you shrink a domain too 
aggressively Linux gets confused about where its memory is going and things 
start to break!

> 5. Does Xen support SMT (simultanious multithreading) in the domU to
>    also have logical processors in addition to virtual processors?
>    Depends it on the physical processor so I would need a multi-core
>    system which also supports SMT? But how would I control that I want to
>    use SMT in some domU's and some not?

SMT is supported where the hardware has it (although it seems to be going out 
of fashion now on x86 boxes so maybe that's not such an issue at  the moment) 
DomU's VCPUs can be set to run on any thread context on the system.

I'm not sure to what extent guests are aware of the SMT so they can use it 
themselves; this would be particularly difficult for them to exploit given 
they could in theory be moved to other logical CPUs at any time.

Running dom0 in a dedicated thread context is useful though.

> 6. Can I really have a dedicated processor core assigned to dom0 or
>    any domU and no other domU will use exactly that physical processing
>    unit? (e.g. to make sure the L2 cache can be used more effective on
>    multi-core CPUs)

Yep.

Cheers,
Mark

> I don't know which of these features have to be in hardware or can be
> realized in software, so please don't blame me :-)
>
>
> Maybe an interesting discussion this whole topic :-)
>
> Looking forward to your answers!
>
>
> Greetz
> Julian
>
>
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-- 
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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