Last week I received a very strange customer inquiry in which they would like to limit the number of physical CPU sockets seen by ESXi. As you can imagine, my interest was piqued as this is usually not the type of request you hear from customers looking to actively reduce the overall computing power of their underlying hardware, especially if they have paid for it. After digging into the details a bit more, it turns out this is related to licensing.
The customer is running an application on a VM which is licensed by the total number of underlying physical CPU sockets of the server, regardless if they are actively being used by the application or not. The vendor shall be left nameless but I am sure some of you can make some educated guesses 🙂 The customer was in the process of performing a hardware refresh where they would be moving from a 4 socket CPU to an 8 socket CPU and they would be negatively impacted by this change from a licensing standpoint. I can understand their concerns, they have now just doubled their application licensing cost without actually benefiting from the actual hardware update.
Unfortunately, after a bit more research, I found that it is not possible to reduce or limit the number of physical CPU sockets from ESXi. The only capability that we do have today around this topic is to limit the number of Logical CPUs that ESXi can see. This capability is exposed as an ESXi Advanced Setting called VMkernel.Boot.maxPCPUS and by default, this is set to unlimited as you would expect. What this setting does is takes the total number of logical CPUs that you wish to expose to ESXi and then evenly distributes that across your physical CPU Sockets as best as it can.
You can change this setting using a variety of methods including the vSphere Web Client, vSphere C# Client, ESXi Embedded Host Client, vSphere API which includes ESXCLI & PowerCLI. One alternative which I have seen some postings online about which is the ability to turn off specific CPU sockets for certain hardware platforms by using the system BIOs, having said that, I have not actually seen any real confirmation that this is in fact possible.
Below are screenshots using the vSphere C# & ESXi Embedded Host.
List the current configurations
esxcli system settings kernel list -o maxPCPUS
Set a new configuration
esxcli system settings kernel list -s maxPCPUS -v 4