[ovirt-users] several ovirt questions - CPU allocation, and use threads as CPU

[Jason Keltz]

On 11/19/2015 06:00 AM, Martin Sivak wrote:
> Hi Jason,
>
>> How do I go about seeing how many CPUs I've allocated across all the VMs on a host?
>> If shared CPUs are possible, can I specify VMs that are allowed to shared CPUs versus
>>   VMs that reqiure dedicated CPUs?
> Unfortunately the UI is lacking in this area. We do not currently have
> any overview page.
I will submit an RFE when I get a chance.

> Also we do not currently support dedicated CPU to VM mapping. You can
> use CPU pinning to select the CPUs that should be used, but the CPU
> can be used for any other VM too (so it is a limitation for the VM
> assignment, not for the CPU assignment).
>
> There might be a workaround for this using the isolcpus
> (http://www.linuxtopia.org/online_books/linux_kernel/kernel_configuration/re46.html)
> kernel boot option and then puting VMs on those CPUs using pinning.
> But we have never tested this.
Ok.
>> What happens if I allocate more cores than what I actually have?
> We won't allow your VM to start if it requires more cores than
> physically available on the destination host.
>
Actually, I think I wasn't clear with my message.  If I have only 4 
cores total, and I try to allocate 5 cores to one VM, I would expect the 
VM wouldn't start.  What I want to understand is what happens when I try 
to run 3 x 4 core VMs on a 4 core system.  By default, they would all 
share the 4 cores?  oVirt would never tell me that I am out of capacity 
as long as the VMs that I'm running are all under the total number of 
cores in the system?

>> If I specify say, a dual CPU system, would that system show up as 4 cores (2 real cores and 2 hyper-threaded cores)
> CPU pinning and core reporting always use thread IDs so it makes no
> difference there.
>
> But the scheduling takes this option into account when computing the
> load or when checking if enough cores are available (useful with
> relation to your previous question).
You didn't mention whether I'm at an advantage to enable "Use Threads as 
CPUs" or to leave it off?
>
> Would you care to open a bug about the CPU pinning overview? It might
> be a nice RFE considering we have something similar for NUMA already.
Sure!

Thanks!

Jason.

On Tue, Nov 17, 2015 at 4:52 AM, Jason Keltz <jason.keltz at gmail.com> wrote:
>> Hi.
>>
>> When I go to the "Clusters" in ovirt engine, I can see the total number of
>> VMs in the cluster.   If I open up "Hosts", I can see how many VMs are
>> running on each host.  I can see the memory consumption as well.  How do I
>> go about seeing how many CPUs I've allocated across all the VMs on a host?
>> What happens if I allocate more cores than what I actually have? If shared
>> CPUs are possible, can I specify VMs that are allowed to shared CPUs versus
>> VMs that reqiure dedicated CPUs?  Finally, I presently have "Use Threads as
>> CPUs" off.  I don't remember if that was the default, or whether I selected
>> that option during installation.  Is there any disadvantage to turning it
>> on?  I realize that those other cores aren't "real" cores - they're the
>> hyper-threading course,  but it's not clear how ovirt would handle those
>> cores.  If I specify say, a dual CPU system, would that system show up as 4
>> cores (2 real cores and 2 hyper-threaded cores) ?
>>
>> Thanks for any help..
>>
>> Jason.
>>
>>
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