NUMA Aware VM Configuration
An administrator observes that a VM with 16 vCPUs on a 2-socket host (8 cores per socket, HT disabled) is experiencing poor performance. The VM's NUMA topology shows 2 vNUMA nodes, but performance is worse than expected. What is the most likely reason?
Quick Answer
The answer is a single virtual socket with 16 cores, which forces vSphere to create one vNUMA node that spans both physical NUMA nodes, causing cross-node memory access. When a VM’s virtual socket count does not match the host’s physical NUMA node topology—here, two sockets with eight cores each—vSphere cannot align the vNUMA nodes properly, so the single vNUMA node must access memory across the physical NUMA boundary, drastically increasing latency. On the VCP-DCV exam, this scenario tests your understanding of NUMA-aware VM configuration, a common trap where candidates assume more vCPUs simply need more cores per socket, ignoring that vNUMA node count mirrors virtual socket count. The key insight is that for optimal performance, the number of virtual sockets should equal the number of physical NUMA nodes, with cores per socket matching cores per node. Memory tip: “Sockets shape NUMA nodes—match them to avoid cross-node loads.”
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VM is configured with a single virtual socket with 16 cores, causing the vNUMA nodes to span both physical NUMA nodes and lead to remote memory accesses.
When a VM is configured with a single virtual socket with 16 cores on a dual-socket host (8 cores per physical NUMA node), vSphere creates 2 vNUMA nodes, each with 8 vCPUs. However, the single virtual socket causes vSphere to interleave or misalign the memory of those vNUMA nodes across both physical NUMA nodes, resulting in cross-node memory accesses and degraded performance. Option B is incorrect because memory reservation does guarantee that physical pages are backed; the statement 'memory is reserved but not backed by physical pages' is false. Option C is incorrect because the virtual hardware version does not affect NUMA topology; the issue is the socket configuration. Option D is incorrect because insufficient RAM would cause memory ballooning or swapping, not the specific cross-node NUMA behavior described.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The VM is configured with a single virtual socket with 16 cores, causing the vNUMA nodes to span both physical NUMA nodes and lead to remote memory accesses.
Why this is correct
A single vNUMA node across physical NUMA nodes forces cross-node memory access, increasing latency.
- ✗
The VM's memory is reserved but not backed by physical pages
Why it's wrong here
Memory reservation ensures pages are pre-allocated; this would not degrade performance.
- ✗
The virtual hardware version is too old
Why it's wrong here
Old virtual hardware might lack some optimizations but is unlikely the primary cause of such a performance drop.
- ✗
The host has insufficient RAM
Why it's wrong here
Insufficient RAM would cause memory contention, but the issue is specifically poor performance despite expected NUMA topology.
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Same concept, more angles
1 more way this is tested on VCP-DCV
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An administrator configures a VM with 4 vCPUs and 32 GB memory on a host with 128 GB RAM and 8 cores (HT enabled). The VM runs a database that requires high memory bandwidth. Performance monitoring shows low CPU usage but high memory latency. Which configuration change is most likely to improve memory performance?
hard- ✓ A.Enable the 'Preferred NUMA Node' VM option
- B.Increase the memory reservation to 64 GB
- C.Disable Hyper-Threading on the host
- D.Increase the number of vCPUs to 8
Why A: Preferred NUMA Node allows the VM to be pinned to a specific NUMA node, reducing cross-node memory access. Option B is wrong; increasing reservation does not improve latency. Option C is wrong; disabling HT reduces overall throughput. Option D is wrong; more vCPUs may increase contention.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This VCP-DCV practice question is part of Courseiva's free VMware certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the VCP-DCV exam.