NUMA and Hyper-Threading (preferHT) Considerations for ESXi Virtual CPU Scheduling

Products

• VMware vSphere ESXi

Issue/Introduction

This article discusses performance and configuration best practices when assigning vCPUs to virtual machines in environments with Hyper-Threading (HT) and NUMA. It addresses questions regarding vCPU count relative to NUMA capacity, the effect of numa.PreferHT=1.  

Environment

VMware vSphere ESXi 8.x
VMware vSphere ESX 9.x

Resolution

• vCPUs spanning multiple NUMA nodes may experience remote memory access latency. Keep a VM’s vCPU count within one NUMA node’s thread capacity whenever possible.
• The numa.PreferHT=1 parameter instructs ESXi to prefer scheduling vCPUs on sibling Hyper-Threads of the same physical core before using additional cores within the same NUMA node.
• For CPU-bound or large VMs, forcing vCPUs to share Hyper-Threads can cause resource contention and NUMA imbalance.

Additional Information

Configure virtual machines to use hyper-threading with NUMA in VMware ESXi