Top 5 VMWare Performance Issues: CPU Over-subscription (CPU Ready Time)

If you are running VMware in your data center, you may be experiencing VMware CPU over-subscription. 33% of all businesses have CPU over-subscription within their VMware hypervisors based on our analysis of 4,600 IT infrastructures.

VMware CPU over-subscription, essentially means, what percentage of time are my VM’s trying to get access to a physical CPU, but it can’t. This is measured by VMware as “CPU Ready Time”. Ideally, ready time should be at 0, but there is going to be some natural over-subscription, but what percentage is considered ok? VMware recommends less than 5% ready time for all VM’s, and a CPU Ready value of 10% or greater on a vCPU indicates declining performance for server workloads (this is much higher than VMware recommends).

Our analytics platform looks at CPU ready time (over-subscription) over two time frames (1) a 20 second period (2) over any given hour of time. When we say that 33% of businesses have CPU over-subscription, we are saying that the CPU has over 5% CPU ready time for an entire hour. That means that 5% of the hour the VM’s CPU had something to process, yet it couldn’t do it, this is a major performance issue.

What does this mean for you? This causes applications to be slow, such as web applications with Java or .NET. When you look for this issue, you will see it measured in milliseconds, you will have to do some math and calculate milliseconds to seconds in order to calculate ready time. To give you an example of how to calculate this: For an hour, if you take the total number of milliseconds for the hour, divide it by 3.6 Million and multiple it by 100, this will give you the CPU ready time for that hour.

Where can you go to get help? I included a good best practices guide that can be found on the VMware community title “Best Practices for Over-subscription of CPU, Memory and Storage in vSphere Virtual Environments”. If you are unsure of how your VMware performance looks like today, we have conducted our analytics on over 4,600 business networks and have a comprehensive method of looking at VMware performance characteristics. Learn more about how our analysis works here.


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