Increasing CPUs in Windows Server 2008 R2 VM

[nizuwizu]

Hi,

I currently have a Windows Server 2008 R2 VM (Citrix Xenapp desktop server) that currently has 2 CPUs assigned but i would like to increase this to 4. Is there any advantage or disadvantage of assigning 4 CPUs or increasing the Cores to 2 per CPU? Does anyone have a preference?

The VM is on a host that is licensed with a 2012 Datacenter license for 2 sockets. I assume this doesn't restrict the VMs to only have 2 CPUs assigned to them and therefor forcing me down the core option? Is there any other licensing gotchas with giving the VM 4 CPUs rather than increasing the cores?

Thanks for help.

[GaneshNetworks]

Its worth to read VMware KB: Setting the number of cores per CPU in a virtual machine

[GaneshNetworks]

VMware says,

"When creating a virtual machine you have the option to specify the number of virtual sockets and the number of cores per virtual socket. In general, we recommend leaving this at the default value of 1 core per socket (with the number of virtual sockets therefore equal to the number of vCPUs).

In some cases, setting the number of cores per virtual socket to the number of cores per physical NUMA node shows a performance improvement".

Source: https://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.1.pdf

[nizuwizu]

Ok so the general suggestion is to use 4 vCPU (1 Core per socket) rather than 2vCPU (2 Core per socket).

Does anyone know where this would leave me with the licensing question?

Thanks

[MKguy]

As far as licensing goes, you're fine with a 4 socket and 1 core per socket configuration.

Windows Server 2008 R2 Standard supports up to 4 sockets, Enterprise up to 8 and Datacenter up to 64:

http://windowsitpro.com/windows-server-2008/q-what-maximum-number-physical-sockets-and-cores-windows-server-2008-r2-supports

The "don't change cores per socket unless you have a good reason like licensing" doctrine is explained in more detail here:

http://blogs.vmware.com/vsphere/2013/10/does-corespersocket-affect-performance.html

[nizuwizu]

Ok great thanks - the link was just what i was looking for. Although i'm still not fully clear on the NUMA nodes. I assume as i have a 6 core processor (x2) with hyperthreading that if the vCPUs i require per VM is 6 or less then wide and flat is not entirely necessary but its a safe rule to apply.

So the license that is applied to the host by CPU sockets is only relevant to the host and doesn't affect the amount of vCPUs i can apply to the VMs, this is only affected by what is supported by that OS.

[MKguy]

Although i'm still not fully clear on the NUMA nodes. I assume as i have a 6 core processor (x2) with hyperthreading that if the vCPUs i require per VM is 6 or less then wide and flat is not entirely necessary but its a safe rule to apply.

As long as

a) Your largest VM by vCPU count fits into a single NUMA-Node (physical core count)

OR

b) You're not exceeding 8 (read: at least 9) vCPUs for a single VM

Then there is no point in worrying about (v)NUMA scheduling.

So the license that is applied to the host by CPU sockets is only relevant to the host and doesn't affect the amount of vCPUs i can apply to the VMs, this is only affected by what is supported by that OS.

Yes, in the case of Windows, it's a hard socket limit per installed server edition. Besides the OS has no way of knowing what license you assign logically to physical host's CPUs.