CPU Oversubsciption Ratio Question

[jmartuto]

Hello All,

I have a silly question. I am trying to calculate a 1:1 vCPU:pCPU ratio, so no CPU oversubscription.  The system has Hyperthreading enabled.

If I have a physical host with 48 physical cores and Hyperthreading is enabled, in order to use all the physical cores (but not oversubscribe), would I size the VM with 48 vCPU or 96 vCPU?

To use all 48 physical cores, I am thinking I should configure the VM with 96 vCPU. If the VM is sized with 48 vCPU, I'm only using half of the physical cores I paid for.

Please let me know. Thanks.

[abhilashhb]

Enabling hyperthreading does not increase the number of physical cores you have on the socket.

What it does is, it lets your run two threads on the same physical core making it look like you have twice the number of logical cores. From what is reported by different sources and for different workloads, you will get a max of 20-30% improvement in performance when you enable hyperthreading.

In your case, if you don't enable it, you will not be using half of the available physical cores, you'll be using all of it.

When you enable hyperthreading, you might see an improved performance if you have multiple VMs competing for CPU resources. Not sure if it will make a lot of difference if you have just one VM. 

[jmartuto]

Thanks for the detailed response Abhilash.

To be honest, I'm not sure I fully understand. 

If I have an ESX host with 48 physical cores and hyperthreading is enabled, I have 96 logical processors and can configure a VM with up to 96 vCPU.

My question is, if I configure a VM with 48 vCPU to match 48 physical cores, am I consuming half of the physical cores or all of them? I can technically max out the VM at 96 vCPU.

From what I understand, with Hyperthreading enabled, vCPU is a logical thread, not a physical one.

Please let me know what I'm missing.

Thanks.