RE: [Xen-users] Xen 3.0 and SMP

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> Questions about the roadmap for 3.0 and SMP?
>  
> Can a domU be a larger SMP than the underlying host, e.g. can 
> one make a 5-way domU on a 4-way host?   

Yes.

> How do SMP domUs get scheduled?   For example, if there are 2 
> 2-way domUs running on a 2-way host, can both domUs be 
> running simultaneously (1 CPU from each) or does each SMP 
> domU get scheduled en mass, using both host CPUs when running 
> at all times?

Both doms could end up getting run at the same time, particularly if one
VCPU in each is idle.
  
> What is the expected loss in performance of running a domU 
> with identical SMP characteristics to its host?   In other 
> words,  if running a single 4-way domU on a 4-way host, what 
> would one expect the performance loss to be?

Depends how much dom0 is being invoked to do IO. It's a real pain if
you've got a 4 way machine with an IO intensive 4-way guest and dom0
running device drivers: you have 5 balls to keep in the air and only 4
hands. You might be better off hot-unplugging one of the guest's VCPUs.
[NB the situation is even worse on a 2-core box]

> Can an SMP domU migrate to a host with less cpus, more cpus?

Yes, they will be able to.

Ian

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