ESXi CPU Ready and vCPU Realities – The Silent Threat to Performance

???? Introduction

One of the most overlooked yet critical performance metrics in virtualized environments is CPU Ready. In this blog post, we’ll explore what CPU Ready actually means, how it relates to vCPU and MHz values in ESXi, and how it can silently degrade performance if not managed properly.

???? What is CPU Ready?

CPU Ready is the amount of time a virtual machine (VM) is ready to run on a CPU but is waiting for the physical host to allocate processing resources.

In simple terms: "Your VM is ready to work, but the host is too busy to let it."

The higher the CPU Ready time, the slower the VM will respond—even if it doesn’t look busy from a traditional monitoring perspective.

???? vCPU vs MHz – The Reality

On VMware ESXi, each vCPU is typically shown with a nominal value of 1000 MHz. However, this is not the actual physical CPU frequency. For example:

• Physical CPU: 2.4 – 2.7 GHz

• A VM with 6 vCPUs → shows 6000 MHz total

• But this is just a scheduler approximation, not guaranteed performance

???? How to Monitor CPU Ready?

You can use SSH to log in to your ESXi host and run:

esxcli hardware cpu list

For live performance data:

esxtop

Then press c to view CPU stats. Focus on the %RD (Ready Time) column:

???? Why Does CPU Ready Increase?

• Too many vCPUs assigned across VMs

• Host is overcommitted or has limited physical cores

• Several high-load VMs running simultaneously

✅ How to Reduce CPU Ready?

• Don’t assign more vCPUs than needed

• Use CPU Reservations for critical VMs

• Enable Latency Sensitivity: High for performance-critical workloads

• Distribute VMs across hosts if possible

???? Real-World Use Case: Apache + PHP-FPM Web Server

Example Configuration:

• 6 vCPUs, 32 GB RAM (VM on ESXi)

• Apache with mpm_event + PHP-FPM

Performance Outcome:

• With low CPU Ready: 2000–3000 concurrent requests per node

• With 3 nodes: 6K–10K requests spread across the cluster

• With high CPU Ready: Even 1000 users can cause timeouts

???? Conclusion

CPU Ready is a hidden performance bottleneck in ESXi environments. While everything may look fine in tools like top or htop, if CPU Ready is high, your VMs are silently choking.

If you’re serious about performance, especially for web, database, or clustered services, you must:

• Monitor CPU Ready regularly

• Keep vCPU assignments realistic

• Use resource reservations strategically

???? Written by: Emrah AK
???? https://eak.llc