CPU Ready Time Metrics — %RDY and Co-Stop
An administrator is troubleshooting performance issues on a vSphere cluster. Which TWO metrics should be monitored to identify CPU ready time contention?
Quick Answer
The correct answer is %RDY and Co-Stop, as these two metrics directly measure CPU ready time contention in vSphere. %RDY captures the percentage of time a virtual machine is ready to execute instructions but must wait for the ESXi host’s CPU scheduler to allocate physical cycles, while Co-Stop tracks time lost when vCPUs within a single VM are forcibly co-scheduled and then descheduled due to contention on the same physical core. On the VCP-DCV exam, this question tests your ability to distinguish between CPU scheduling metrics and common red herrings like %CSTP or %WAIT; a frequent trap is confusing Co-Stop with general CPU ready time, but remember that Co-Stop specifically applies to SMP VMs with multiple vCPUs. For a quick memory tip, think “Ready waits for any core, Co-Stop waits for its twin.”
⚠ Common exam trap
Candidates often confuse CPU ready time with memory or storage metrics, especially since high CPU ready time can manifest as general VM slowness, leading them to incorrectly select Disk Kernel Latency or Memory Swap In Rate instead of the correct CPU-specific counters.
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
✓
Co-Stop
CPU ready time contention occurs when a virtual machine is ready to execute instructions but the ESXi host's CPU scheduler cannot immediately allocate physical CPU cycles. The %RDY metric directly measures the percentage of time a VM is waiting to be scheduled on a physical CPU, while Co-Stop specifically tracks time lost when vCPUs in a single VM are forcibly co-scheduled and then descheduled due to contention on the same physical core. Both metrics are primary indicators of CPU scheduling pressure.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Disk Kernel Latency
Why it's wrong here
Measures storage latency, not CPU.
- ✗
Memory Swap In Rate
Why it's wrong here
Indicates memory overcommitment, not CPU.
- ✓
Co-Stop
Why this is correct
Measures time vCPUs are co-scheduled but waiting due to HT.
- ✗
Network Packet Drop Rate
Why it's wrong here
Indicates network issues, not CPU.
- ✓
%RDY (CPU Ready)
Why this is correct
Directly measures CPU ready time.
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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. A vSphere administrator is troubleshooting a VM that hosts a critical database. The VM is configured with 8 vCPUs and 64 GB memory on an ESXi host with two 10-core CPUs (hyperthreading enabled). Performance charts show CPU ready time averaging 12% during peak hours, and the database application is experiencing high transaction latency. The administrator has verified that no other VMs are contending for CPU. Which action will most likely reduce CPU ready time without negatively impacting database performance?
medium- A.Disable hyperthreading on the host.
- B.Assign a CPU affinity mask to pin the VM to specific physical cores.
- C.Increase the VM's memory reservation.
- ✓ D.Reduce the number of vCPUs assigned to the VM to 4.
Why D: Reducing the number of vCPUs to 4 reduces CPU scheduler overhead and can lower CPU ready time when the VM has more vCPUs than needed. With 8 vCPUs on a host with 20 physical cores (40 logical with HT), and 12% ready time, the VM is likely oversubscribed for its workload. Many database applications do not scale linearly beyond 4 vCPUs, so reducing to 4 often improves performance by decreasing contention and improving cache locality. Option A is wrong: disabling hyperthreading reduces logical CPUs, potentially increasing contention. Option B is wrong: CPU affinity can lead to unbalanced load and is not recommended in DRS clusters. Option C is wrong: increasing memory reservation does not affect CPU ready time.
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.