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RE: [Xen-users] RE: xm vcpu-set command has no effect on fully virtualized machine

  • To: "Sangati, Ramya Govinda Reddy" <RamyaGovinda.Sangati@xxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
  • From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • Date: Tue, 24 Apr 2007 11:25:14 +0200
  • Delivery-date: Tue, 24 Apr 2007 02:24:10 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: AceGPf6kYT7Ej0ZnT4Ofsm/E6KGpPwAAc89QAAQ3E7A=
  • Thread-topic: [Xen-users] RE: xm vcpu-set command has no effect on fully virtualized machine


> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Sangati, Ramya Govinda Reddy
> Sent: 24 April 2007 08:15
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] RE: xm vcpu-set command has no effect on 
> fully virtualized machine
> Hi,
> I tried the command 
> xm vcpu-set <domain ID> < vcpu-count> 
> in Red Hat 5, to reduce the number of virtual CPU's allotted 
> to a virtual
> machine. I am encountering a problem in this. 
> When I install a guest OS using Paravirtualization option the 
> above command
> works fine. 
> However when I execute the above command for a guest OS which is fully
> virtualized the command has no effect. Is this expected? Or 
> is it a bug?

Well, I guess we should say it's a "feature" as in "that's not a bug,
it's a feature". Now for that "real" answer: The reason that this
doesn't work is that there's no REAL (standardized) way to tell an OS
that there's suddenly a different number of processors. If the a system
supports CPU hotplug at all, it will rely heavily on firmware (BIOS or
whatever) as well as special hardware in the system, and probably also
special drivers for the hotplug functinality for any particular OS that
supports CPU hotplug. 

> When I type the command xm list, for a par virtualized OS, it 
> shows the
> changes that I made, whereas for a fully virtualized OS, 
> there is no change
> in the number of VCPU's.   
> Also xm mem-set <domain ID> < memory> does not work in a 
> Fully Virtualized
> Guest OS. 

Yes, the number of VCPU or bytes of memory owned by guest is maintained
by the hypervisor itself. The problem lies in telling the  OS that it's
just gained/lost some of that. 

> Regards,
> Ramya.S.G.

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