Xen users - Sep 2008 - More vcpus than physical cores and hvm

Hi List,

  I have a server with two QuadCore 2.93Ghz processors and 16GB Ram,
running Debian etch and Xen 3.2-1.

  Until yesterday I had in this server 2 hvm virtual machines, running
w2k, 4 vcpus each without any problem, the use of each virtual machine
rarely reach 70%. Today I cloned one of the virtual machines and
started they with 2 vcpus, right after I start this new guest the use
of cpus on the others guest vms showed by task manager climb to 50%
(kernel time)  and If I start another new guest with 2 vcpus, the
first two guests reach close to 100% of cpu use

  Can I have more vcpus than physical cores on Xen ?? There''s
something to do with hvm ? Have some configuration (cpu scheduling) to
improve this situation ? Someone have a similar setup ?

 Best Regards,

Alexandre

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On Wed, Sep 24, 2008 at 4:32 PM, Alexandre Biancalana
<biancalana@gmail.com> wrote:
> Can I have more vcpus than physical cores on Xen ??
sure, if you count Dom0, you already had more vcpus than cores.

i''d guess that what you''re seeing is not a problem; but a
limitation
of language.

imagine an extreme case, machine with few cores and many DomU''s.  the
hypervisor CPU scheduler would assign a fraction of CPU to each DomU,
so they run slower.  in these conditions nobody would be surprised
that even light loads can go to 100% cpu use.

your case is nowhere extreme; but similar. before you had unused
capacity; but it was assigned to the DomU''s, so they report only 70%
use.  when you start using that extra capacity, the ''old''
DomU''s get a
smaller slice, so they use a bigger percentage of that smaller slice.

-- 
Javier

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