Re: Question about vcpu_avail

[Michal Novotny <minovotn@redhat.com>]

On 11/23/2009 04:28 PM, Keir Fraser wrote:
> On 23/11/2009 15:02, "Michal Novotny"<minovotn@redhat.com>  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
> pseudocode):
>   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
>
>    

Well, I don't know Keir since the main use scenario for maxvcpus is to 
set number of vcpus for the guest so a new definition in Xend should be 
required.

Then you just issue `xm vcpu-set {domainName} {numberOfVcpus}` command 
to change number of VCPUs used by the guest, method `setVCpuCount(self, 
vcpus)` from XendDomainInfo.py is called to set number of VCpus so check 
should be implemented here as:

      def setVCpuCount(self, vcpus):
         if vcpus > self.info['maxvcpus']:
             log.debug("Cannot set %d vcpu(s) for domain %s (domid %d)."
                       "Maximum number of vcpu(s) is set to %d" %
                                 (vcpus, self.info['name'], self.domid,
                                  self.info['maxvcpus']))
             raise VmError("Cannot set %d vcpu(s). Maximum number of 
vcpus for "
                           "this domain is %d" % (vcpus, 
self.info['maxvcpus']) )

          self.info['vcpu_avail'] = (1 << vcpus) - 1
          self.storeVm('vcpu_avail', self.info['vcpu_avail'])

And domain_max_vcpus() call should be changed from 
`xc.domain_max_vcpus(self.domid, int(self.info['vcpus']))` to 
`xc.domain_max_vcpus(self.domid, int(self.info['maxvcpus']))`.

The only change that should be done to xm/create.py is to add `maxvcpus` 
to add_conf mapping to allow it to be read/parsed out from the config file.

Well, I've been looking to the code and there is self.info['VCPUs_max'] 
in XendDomainInfo.py. What is it used for? Could it be used for the same 
purpose like `maxvcpus` from example above?

Michal

>> 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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