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Re: [Xen-users] One or two vcpu?

On Fri, 2010-03-26 at 15:19 +0100, Markus Hochholdinger wrote:
> Hello,
> Am 26.03.2010 um 13:35 Uhr schrieb Peter Privat <peter.privat@xxxxxxxxxxxx>:
> > On Fri, 2010-03-26 at 12:24 +0000, Rob MacGregor wrote:
> > > > Does it make any difference to the performance for the domU when vcpu
> > > > is increased to use 2 cpu:s?
> > > That rather depends on what you're doing with that domU.
> > Sure! But I wondered if they had any real effect or were just there for
> > satisfying software or OS:es that may require more than 1 core or
> > whatever. Will the adding of one more virtual CPU (vcpu) have the same
> > performance effect as like adding one more physical core to a physical
> > hardware? (upgrading a real MB with 1 core cpu to a 2 core cpu).
> it depends if you have real cpus! Say, you have 4 cores on your hardware, 
> giving 4 vcpus to your domU will result in maximum performance for this domU 
> on this hardware. But if you assign more than 4, performance may decrease. If 
> you assign only 1 vcpu to your domU, this domU will only use one cpu at a 
> time.
> I assume the software in your domU will scale over all available cpus.
> Hope this helps,

Thank you Markus,

That's the answer I was looking for. Changing the amount of vcpus will
actually have a real effect. I was just wondering if it was a cosmetic
thing for the domU OS or its applications. And of course, giving more
virtual cpu:s that I got physical, will then decrease performance.
Great addition to the knowledge institute! :)


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