[Engine-devel] CPU Overcommit Feature

[Dennis Jacobfeuerborn]

On 12/17/2012 07:13 PM, Simon Grinberg wrote:
> 
> 
> ----- Original Message -----
>> From: "Greg Padgett" <gpadgett at redhat.com>
>> To: "engine-devel" <engine-devel at ovirt.org>
>> Sent: Monday, December 17, 2012 4:37:57 PM
>> Subject: [Engine-devel] CPU Overcommit Feature
>>
>> Hi,
>>
>> I've been working on a feature to allow CPU Overcommitment of hosts
>> in a
>> cluster.  This first stage allows the engine to consider host cpu
>> threads
>> as cores for the purposes of VM resource allocation.
>>
>> This wiki page has further details, your comments are welcome!
>> http://www.ovirt.org/Features/cpu_overcommit
> 
> Basically looking good.
> Hyperthread though is vendor specific. 
> 
> For AMD it's Clustered Multi-Thread while for Intel it's Hyper-Thread 
> Official name is  simultaneous multithreading (SMT) but no one outside of the academy will recognize that.
> 
> in libvirt if I read it right it's <attribute name='thread_siblings'>
> 
> So why not just call it threads.
> We'll have cpuSockets, cpiCores, and cpuThreads, should be clear when in CPU context.
> 
> In the GUI just change hyperthreads to CPU threads. While in the tool tip explain that it's either AMD Clustered Multi-Thread or Intel Hyperthread 

Does this affect only the number of potential vCpus for the guests or does
this also have an impact on the actual scheduling?
So far I always disabled HT out of fear that a 2 vCpu guest might actually
be scheduled to run in 2 threads of the same core but now I'm not so sure
anymore. In the HT case does KVM know that two threads belong to the same
core and will it only schedule its vCpus on distinct cores? Is there some
documentation about this somewhere?

Regards,
  Dennis