[Engine-devel] CPU Overcommit Feature

[Dennis Jacobfeuerborn]

On 12/18/2012 06:33 PM, Andrew Cathrow wrote:
> 
> 
> ----- Original Message -----
>> From: "Dennis Jacobfeuerborn" <dennisml at conversis.de>
>> To: engine-devel at ovirt.org
>> Sent: Tuesday, December 18, 2012 12:30:34 PM
>> Subject: Re: [Engine-devel] CPU Overcommit Feature
>>
>> 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?
> 
> This is about the maximum number of vCPUs we can give to a VM.
> If the machine has 32 Physical cores that are hyperthreaded then do we say the max number of vCPUs for a single VM is 32 or 64.

If the actual scheduling of vCPUs cannot distinguish between threads and
cores then why would you even want to limit yourself to 32 in you example?
In that case the scheduling might end up being inefficient no matter how
many vCPUs you assign to a guest so why restrict the user? (You might of
course want to limit the user for policy reasons but that has nothing to to
with the thread/core topic.)

On the other hand if KVM does only schedule the vCPUs on distinct cores and
extending the count from 32 to 64 implies that this distinction is to be
disabled then this will have a performance impact for the guest.
In that case I might want to limit the guests to just the 32 physical cores
so two vCPUs of a single guest don't get scheduled on two threads of the
same core.

I've never really looked that closely into the scheduling issue but it
might play a role here so I asked if someone had any pointers to infos
about how exactly KVM makes its scheduling decisions.

Regards,
  Dennis