Mailing List Archive

Xen 3.0 and SMP
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? If not, what are the constraints on the #CPU for an SMP domU.

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?

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?

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

Alan
RE: Xen 3.0 and SMP [ In reply to ]
> 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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