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RE: [Xen-users] vcpus higher than real cpus possible?

Same vcpu’s or less than real cpu’s for performance.


Having more vcpu’s assigned to a domu than real cpu’s is not a supported scenario, although it does seem to work.  The scheduling of such a configuration is a nightmare though, and your guest system will be interrupting itself when it believes concurrent processing is taking place, which could (will?) lead to instability.  You would likely see more cpu time spent on state switching than actual work in such a setup.


From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of David Hlácik
Sent: May 17, 2008 3:51 PM
To: Stefan de Konink
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] vcpus higher than real cpus possible?


But best approach is to use VCPUs same as real CPUs to achieve best performace right?


On Sat, May 17, 2008 at 10:32 PM, Stefan de Konink <skinkie@xxxxxxxxx> wrote:

On Sat, 17 May 2008, [ISO-8859-2] David Hláèik wrote:

> Hello, i have just migrated machine to xen server where only 2 cpus are
> available. I have copied config from machine where domU was located before.
> That machine had 4 CPUS (QUADCORE), current machine has 2 CPUS (1 XEON
> 2cores).
> I have forgot to change vcpus from 4 to 2 in config, but .. what really
> surprised me ... machine started . How it is possible? It was really slow
> alltought and not possible for working (Windows server 2003).

Since Xen is always doing this :) Think for one processor having 2 DomU's
running. Technically you have then 3 vcpu's on the same moment. It is just
about scheduling tasks for a vcpu.



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