[CentOS-virt] What are KVM guest cores?

MargoAndTodd margoandtodd at gmail.com
Thu Nov 18 18:32:16 EST 2010

On 11/18/2010 01:52 PM, Kenni Lund wrote:
> 2010/11/18 MargoAndTodd<margoandtodd at gmail.com>:
>> Hi All,
>> What are guest cores in KVM?  Are they fake, like everything else
>> in the guest?
> KVM requires VT-x support in your CPU, to get optimal CPU-performance
> in the guest. Eg. no, it is not emulated like for example the sound
> card - the virtual CPU uses the hardware virtualization support in
> your CPU. How is is shown in the guest (eg. the name of it and its
> capabilities/flags), depends on the options you give to KVM. If you
> start KVM with the -cpu host argument, your Core i7 cores will get
> presented exactly as Core i7 cores within the guest, with all the same
> capabilities as on your host. This is not done by default, as it would
> make it impossible to migrate from a server with CPU A to another
> server with CPU B, since the CPUs need to be identical (eg. have the
> same flags). Therefore the default virtual CPU in most setups is "QEMU
> CPU vx.xx", which uses the most common CPU capabilities.
>>   Just another process running on the host emulating
>> a core?
>> Or are the guest cores actually connected directly to the physical
>> cores on my motherboard?
> No, they are not directly connected, they are threads on your host.
> Just FYI, you *can* start a guest with more virtual cores than you
> have physically, but you'll see a huge performance hit if you do so
> (due to some internals of KVM). However, you can still exceed your
> physical number of cores, and get fair performance, as long as it's
> spread among several machines. Eg. if you have 2 cores, you can run 2
> (or more) guests with 2 CPUs each, but performance will suffer if you
> run 1 guest with 4 cores.
> Best regards
> Kenni

Thank you.  I am slowly starting to get it.

Do you have a rule of thumb as to how many core to assign
to a guest?  For instance, with an Intel x5650 with 6 real
and 12 hyperthreaded cores, how many cores would you assign
to the guest?

Many thanks,

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