Many people have confusion sight about how really a host assigns CPU resource to the virtual machines; More precisely I can say how the processing operation of a VM has been executed via the physical CPU resources. In the Intel terminology, the physical processor is a CPU socket, but in this post, I consider the pCPU as a physical core in the existing sockets of servers.
By default, each of the added vCPU to the VMs is assigned to one of the existing pCPUs. So if we configure 8 vCPU for a VM, there must exist at least 8 pCPU in the host. In other words, if there is not enough pCPU for the VM, it cannot be started.
Based on design, VMware ESXi can handle the CPU oversubscription (request of vCPU more than existing processors/pCPU). It means the pCPU~vCPU ratio is not one by one (1:1) anymore. In the vSphere environment, the ESXi host will handle the processing operations to execute requests of every VM, then the host needs to schedule processing time for each of them. But here the question is what ratio should be configured as the best settings? The answer depends on choosing Capacity or Performance aspects, really it can be very different based on the virtualized application requirements ...
Each VM needs the pCPU resources, then implementation of many VMs specially highly-applicable and resource-consumption virtual machines demand more CPU clocking. So if you provision more VMs and also increase the pCPU~vCPU Ratio (1:2, 1:4 or greater) the performance of the ESXi host will be affected.
As the VMware mentioned vSphere ESXi scheduling mechanism prefers to use the same vCPU-to-pCPU mapping to boost performance through CPU caching on the socket. If there is no specific documentary for the CPU design of the Application, you can set it up with a single vCPU, then scale up based on requires. So oversubscription will not have a serious negative impact.
Also, we must consider the CPU Ready Time is another important metric as the CPU utilization metric is. Generally, vCPU~pCPU ratio is based on many factors like the following:
- Version of ESXi host. Each newer version supports more ratio.
- Supported features and technologies by physical processor.
- Workload rates of critical Applications that are implemented in the virtual environment.
- The capacity of existing processor resources in other members of the cluster and their current performance, especially when we require a higher level of hosts fault tolerance in the virtualization infrastructure. Available resources in the cluster will specify each VM that can be placed on which host in front of a host failure.
Should we use Hyperthreading or not ?!
Hyperthreading is a great technology that makes a single pCPU act as the two logical processors. In the case of the low-usage of ESXi host, each of those logical cores can handle two independent applications at the same time. So if you have 16 logical processors in the ESXi host, after enabling of HT (In both of the BIOS config and ESXi advanced settings) you will see the host has 32 logical processors. But using HT does not mean performance is increased always and it's highly dependent on application architecture. So in some cases maybe you encounter performance degradation via HT usage. Before enabling of HT in the ESXi hosts, review critical virtualized applications deploy on their VMs.
Source of original post in my personal blog: Undercity of Virtualization: Virtualization Tip1: Relation between physical CPU & virtual CPU