On Mon, Jun 5, 2023, 21:52 David White <dmwhite823@protonmail.com> wrote:
These are Dell R630s with 2 physical CPUs.

Intel(R) Xeon(R) CPU E5-2640 v3 @ 2.60GHz
CPU Cores per Socket:
8

Ooooh, so even though there are 32 logical cores, the physical CPUs are quad core.

Is there a downside to enabling "Count Threads As Cores" in the cluster configuration?
Why would that not be enabled by default?

Hello,

SMT threads are not identical to physical cores, as two threads essentially share the same core processing pipeline and cache, increasing the utilization of the physical core resources.

In reality the performance will vary wildly between 1.0 to 1.5+ of that of a single physical core.
In short,  if you really need more than 16 active vCPUs, you may get lower performance depending on your exact workload.

Gilboa



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------- Original Message -------
On Monday, June 5th, 2023 at 2:22 PM, Gilboa Davara <gilboad@gmail.com> wrote:

Hello,

Hardware?
As far as I remember you cannot assign more vCPU than the number of physical cores you have, unless you enable "Count Threads As Cores" in the cluster configuration, and even than, the number of vCPUs is limited to the number of SMT threads you have.

- Gilboa

On Sun, Jun 4, 2023 at 6:17 PM David White via Users <users@ovirt.org> wrote:
I have a fully patched / up-to-date engine:
Software Version:4.5.4-1.el8

And a fully patched, up-to-date host.
[root@cha3-storage dwhite]# yum info ovirt-host
Last metadata expiration check: 1:33:40 ago on Sun 04 Jun 2023 09:28:39 AM EDT.
Installed Packages
Name : ovirt-host
Version : 4.5.0
Release : 3.el8
Architecture : x86_64
Size : 11 k
Source : ovirt-host-4.5.0-3.el8.src.rpm
Repository : @System
From repo : centos-ovirt45

The host has 32GB of RAM, and there's only 1 VM on this host.
When I try to add more CPUs to the VM from the manager UI, I get the following error:
  • The requested number of vCPUs is not available on the host the VM is running on
What's going on here, and why can I not add more vCPUs to this VM?

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