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Re: [Xen-devel] Question about vcpu_avail

To: Michal Novotny <minovotn@xxxxxxxxxx>, "'xen-devel@xxxxxxxxxxxxxxxxxxx'" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Question about vcpu_avail
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 23 Nov 2009 15:28:53 +0000
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On 23/11/2009 15:02, "Michal Novotny" <minovotn@xxxxxxxxxx> wrote:

> I would like to ask about vcpu_avail variable in XendDomainInfo.py. Is
> that variable working and what is it used for ? I think it's a bitmask
> showing what VCPUs are used and what are not, am I correct?

Yes, it allows you to configure which VCPUs are brought online by default on
guest bootup. If you do add a maxvcpus config item, it can be parsed out
into vcpus and vcpu_avail entirely within xm's create.py (excuse dodgy
 if defined(maxvcpus):
     vcpu_avail = (1 << vcpus) - 1
     vcpus = maxvcpus

i.e., no need for xend itself to know about the new config item.

 -- Keir

> Also, I'm 
> thinking of adding some variable of `maxvcpus` to define maximum number
> of vcpus for PV guests. This way we could define a config file like:
> vcpus = 2
> maxvcpus = 4
> and by setting `xm vcpu-set {domain} {numberOfVCPUs}` we can set up to 4
> vcpus. Current behavior is to set to 1 vcpu and back to 2 vcpus but
> never to cross the number defined in of vcpus. Now we could be able to
> allocate up to 4 vcpus to the guest and not just 2... What do you think
> about that idea?

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