High CPU Ready with Low CPU Utilization?

I have noticed an increasing amount of search engine terms which results in people accessing my blog similar to

* High CPU Ready Low CPU usage
* CPU ready and Low utilization
* CPU ready relationship to utilization

So I wanted to try and clear this issue up.

First lets define CPU Ready & CPU Utilization.

CPU ready (percentage) is the percentage of time a virtual machine is waiting to be scheduled onto a physical (or HT) core by the CPU scheduler.

CPU utilization measures the amount of Mhz or Ghz that is being used.

Next to find out how much CPU ready is ok, check out my post How Much CPU ready is OK?

CPU Ready and CPU utilization have very little to do with each other, high CPU utilization does not mean you will have high CPU ready, and vice versa.

So it is entirely possible to have either of the below scenarios

Scenario 1 : An ESXi host has 20% CPU utilization and VMs to suffer high CPU ready (>10%).
Scenario 2: An ESXi host has 95% CPU utilization and VMs to have little or no CPU ready (<2.5%)

How are the above two scenarios possible?

Scenario 1 may occur when

* One or more VMs are oversized (ie: not utilizing the resources they are assigned)
* The host (or cluster) is highly overcommited (either with or without right sized VMs)
* Where power management settings are set to Balanced / Low Power or custom

Scenario 2 may occur when

* VMs are correctly sized
* The ESXi hosts are well sized for the virtual machine workloads
* The VM to host ratio has been well architected

So the question on everyone lips, How can high CPU ready with Low CPU utilization be addressed/avoided?

If you have a situation where you are experiencing high CPU ready and low ESXi host utilization the following steps should be taken

* Right size your VMs

This is by far the most important thing to do. I Recommend using a tool such as vCenter Operations to assist with determining the correct size for VMs.

* Ensure your hosts/clusters are not excessively overcommited

I generally find 4:1 vCPU overcommitment is achievable with right sized VMs where the avg VM size is <4 vCPUs. The higher the vCPU per VM average, the lower CPU overcommitment you will achieve.)
If you have an average VM size of 8 vCPUs then you may only see <1.5:1 overcommitment before suffering contention (CPU ready).

* Use DRS affinity rules to keep complimentary workloads together
VMs with high CPU utilization and VMs with very low CPU utilization can work well together. You  also may have an environment where some servers are busy overnight and others are only busy during business hours, these are examples of workload to keep together.

* Use DRS anti-affinity rules to keep non-complimentary workloads apart

VMs with very high CPU utilization (assuming the high utilization is at the same time) can be spread over a number of hosts to avoid stress on the CPU scheduler.

* Ensure your ESXi hosts are chosen with the virtual machine workloads in mind
If your VMs are >=8vCPUs choose a CPU with >=8 cores per socket and more sockets per host, like 4 socket hosts as opposed to 2 socket hosts. If the bulk of your VMs are 1 or 2 vCPUs, then even older 2 socket 4 core processors should generally work well.

* Use Hyperthreading
Assuming you have a mix of workloads and not all VMs require large amounts of cores and Ghz, using hyper threading increases the efficiency of the CPU schedulure by effectively doubling the scheduling opportunities. Note: A HT core will generally give much less than half the performance of a pCore.

* Use “High Performance” for your Power Management Policy

The above seven (7) steps should resolve the vast majority of issues with CPU ready.

For an example of the benefits of right sizing your VMs, check out my earlier post – VM Right Sizing , An example of the benefits.

Also please note, using CPU reservations does not solve CPU ready, I have also written an article on this topic – Common Mistake – Using CPU reservations to solve CPU ready

I hope this helps clear up this issue.

How much CPU ready is OK?

I have noticed a lot of search results hitting my blog asking

Question: How much CPU ready is OK?

so I thought I would address this question with a quick post.

Of course the answer is it depends, for example Server workloads have a lower tolerance to CPU ready than desktop workloads but as a rule of thumb, here is my thoughts.

For Production server workloads

<2.5% CPU Ready
Generally No Problem!

2.5%-5% CPU Ready
Minimal contention that should be monitored during peak times

5%-10% CPU Ready
Significant Contention that should be investigated & addressed

>10% CPU Ready
Serious Contention to be investigated & addressed ASAP!

In my experience, the above have been good for a rule of thumb.

However, applications which are latency sensitive may be severely impacted even with low levels of CPU ready, these types of VMs should be on clusters with lower CPU overcommitment, leverage DRS rules to separate the contending workloads or in extreme cases, dedicated clusters.

On the flip side, Some servers are much more tolerant to CPU ready, and 5%-10% CPU ready or higher may not noticeably impact performance.

Keep in mind that setting CPU Reservations does not solve CPU Ready, see my post on the topic for more details.

VMware vCenter Operations is a tool which can help easily identify contention (including CPU) within your vSphere environment.

For Virtual Desktop workloads, what level of CPU ready is acceptable will largely depend on the individual user (ie: Power User verses Task Worker). Keep in mind virtual desktop deployments generally have high CPU consolidation ratios of  around 6:1 all the way to >12:1.

I would suggest the following , again as a rule of thumb

<5% CPU Ready
Generally No Problem!

5%-10% CPU Ready
Minimal contention that should be monitored during peak times

>10% CPU Ready
Contention to be investigated & addressed where the end user experience is being impacted.

Any Higher CPU ready will likely be impacting your users, and should be investigated.

VMware have recently released vCenter Operations for View, which you could use to monitor your VMware View environment.