topic Re: VM cpu Cores in ESXi Discussions

VM cpu Cores

joeflint — Tue, 30 Jul 2013 08:24:47 GMT

Hi, is it possible to allocate a VM CPU cores, e.g. assume a VM has 2vCPU and 2 cores per CPU - is this possible. and whats the benefit rather than say allocate 4vCPU?

Re: VM cpu Cores

a_p_ — Tue, 30 Jul 2013 10:30:16 GMT

Yes, that's possible. Some operating systems do not support a large number of sockets (processors) but allow multi-core processors. If you e.g. need 4 vCPUs for an OS which only supports 2 sockets, you need to present virtual multi-core vCPUs to the guest. Another reason for multi-core vCPUs could be per-socket licensing for applications.

André

Re: VM cpu Cores

ShadyMalatawey — Tue, 30 Jul 2013 10:43:53 GMT

Hi...

In VM settings you can assign any number of vCPU (sockets) and how many cores per socket..
Its benefit is for VMs with Guest OS charged for certain number of sockets..
And I think it will differ in very heavy-load VMs in performance..
I'll try to get u the article i passed with once about that...

Re: VM cpu Cores

TomHowarth — Tue, 30 Jul 2013 10:44:06 GMT

Yes with all versions of ESX from 4.0 upwards you can.Firstly it depends upon version, on how you have to do this,

with version 4.x you have to manaully edit the VMX file

with version 5.x you can set the core count from the settings form.

as to why you would do this

it can reduce license count on those applications that are licensed on Sockets

Re: VM cpu Cores

joeflint — Tue, 30 Jul 2013 11:05:36 GMT

Sorry one more question to get my head around this.

I want to make sure that we use no more than 6 vCPU per CPU on ESXi host. So if I had a VM say with 2vCPU and 2 Cores, will this equate to 2 vCPU or 4 vCPU?

Re: VM cpu Cores

a_p_ — Tue, 30 Jul 2013 11:08:04 GMT

2 vCPUs with 2 virtual cores each equals 4 vCPUS. It's only the presentation to the guest which is different by setting up cores per vCPU.

André

Re: VM cpu Cores

ShadyMalatawey — Tue, 30 Jul 2013 11:10:44 GMT

equate to 4 vCPUs

Re: VM cpu Cores

TomHowarth — Tue, 30 Jul 2013 11:12:08 GMT

As Andre says, A guest with 2vCPU and 2cores will use 4 logical CPU on the host. as will a guest with 1vCPU and 4 cores, or a Guest with 4vCPU.

all that has changed is how it is presented to the Guest OS. personally if I am playing with Cores I tend to keep vCPU count as low as possible, this has the added advantage of lowering Socket based licensing costs

Re: VM cpu Cores

Viewaskew — Fri, 23 Aug 2013 01:18:38 GMT

I just had this discussion with someone who was insistent that assigning a VM, for example, with 1 socket and 4 cores that the vCPU was “bound” to one physical CPU (socket not logical) and running on 4 of its cores which is not the case. Then came the question of scheduling and it if it differed if say you had 1 socket and 4 cores or 4 sockets with 1 core each.

As far as Ive been able to research, I can’t find any scheduling differences in either configuration as the vCPU count is the same and as we know, a vCPU is a logical CPU and that’s defined with another example of a server with 2 sockets with 8 cores so has 16 cores or logical processors and if you have hyperthreading enabled this is doubled to 32 logical processors.