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

[Martin Sivak]

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.

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.

> 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.

> 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).


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.

Best regards

Martin Sivak
SLA / oVirt


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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