Sockets vs Cores vs vCPUs

[MAMProgr]

Hi,

What are the differences between Sockets, Cores, and vCPUs.

For example,
a. 4 Sockets, 1 Core, 1 vCPU.
b. 1 Socket, 4 Cores, 1 vCPU.
c. 1 Socket, 1 Core, 4 vCPUs.
are these 3 VMs have same performance?

 

[Vladimir Bulgaru]

Hey!

• Socket is the physical slot for a CPU. It matters if, for instance, you need NUMA (allocated RAM per CPU, which speeds up computation).
• Core - is the physical core a CPU has. Here it gets a bit confusing - within Proxmox the number of cores equals the number of threads, so if your CPU supports hyper-threading, the cores count is double the physical cores of the CPU. Within Proxmox settings, it tells how many threads per each socket should be allocated.
• vCPU - by default, it's the product of sockets and cores per socket allocated. To cite the docs:

  In Proxmox VE the maximal number of plugged CPUs is always cores * sockets. To start a VM with less than this total core count of CPUs you may use the vpus setting, it denotes how many vCPUs should be plugged in at VM start.

  As far as i understand, you can start a VM with less vCPUs than set via sockets and cores and increase them upon need (ballooning style).

Given your scenarios:

1. 4 Sockets, 1 Core, 1 vCPU means that a max of 4 vCPUs are possible, but you are starting a VM (only Linux support) with one vCPU
   
2. 1 Socket, 4 Cores, 1 vCPU - same situation - max of 4 vCPUs are possible, but you are starting a VM with one vCPU
   
3. 1 Socket, 1 Core, 4 vCPUs - not possible to set it up, as vCPUs are <= sockets * cores.

At the same time:

1. 4 Sockets, 1 Core, 4 vCPUs - allows for NUMA optimisations if your server has actual 4 sockets
   
2. 1 Socket, 4 Cores, 4 vCPUs - identical to previous scenario, if NUMA is not used / considered, but may influence the cost of licensing

To sum up:

• most likely you don't need to touch vCPU settings
• more sockets with less cores each (but <= than physical sockets on the system) allows for NUMA optimisation
• less sockets with more cores each may decrease the licensing costs, since some licenses are socket-based

 

[MAMProgr]

Thank you so much for your detailed explanation.