A virtual machine (VM) CPU is not a physical, silicon-based CPU but rather a logical construct managed by the hypervisor. It represents a portion of the real CPU's resources allocated to the VM. Here's how it works and why adding more virtual CPUs (vCPUs) can sometimes degrade performance:
Real CPU - A physical processor core made of silicon that performs actual computation tasks.
VM CPU (vCPU) - A virtualized representation of a physical CPU core, created by the hypervisor, that a VM uses to execute its operations.
Each vCPU operates as a thread or process on the host system, competing for physical CPU time alongside other VMs, system processes, and the hypervisor itself.
Scheduling - The hypervisor schedules each vCPU as a separate thread or process on the physical CPUs. A vCPU doesn't execute directly on hardware; instead, it "waits" for the hypervisor to assign it time on a physical CPU.
Sharing Resources - Multiple vCPUs from different VMs may share the same physical CPU, which can create contention and scheduling delays.
Adding more vCPUs to a VM doesn't guarantee better performance and can, in some cases, negatively affect it due to the following reasons:
Each additional vCPU requires the hypervisor to find more physical CPU cores (or threads) to schedule them. If the physical CPU resources are limited or highly utilized, the vCPUs might experience scheduling delays.
For example, a VM with 4 vCPUs needs 4 physical cores or threads to be available simultaneously for execution. If this condition isn't met, the VM has to wait, causing performance degradation.
More vCPUs increase the hypervisor's scheduling and management overhead. This can reduce overall system efficiency, especially if multiple VMs are competing for the same physical CPU resources.
Comentários