Tag Archives: performance problems

High CPU Ready with Low CPU Utilization?

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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.

Common Mistake: Using CPU reservations to solve CPU Ready

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One of the more common problems I see in virtual environments is over sized virtual machines which typically results in lower performance, and your guessed it, high CPU Ready.

What is CPU Ready?

CPU ready is basically the time it takes a VM to be scheduled onto physical core after it is placed in the CPU scheduling queue.

What is High CPU Ready?

In my opinion, during peak load, anything above 2% (or 400ms) is a concern and should be monitored. Above 5% will be impacting performance (resulting in lower CPU utilization) and 10% or more, should be considered a serious problem and remediated immediately.

The below is a screenshot showing CPU ready from a recent test I conducted in my home lab

To calculate the percentage of CPU Ready, we divide the VMs “Summation” value (in the screen shot above its the “W2K8 CPU TEST VM 1” line by 20000 (ms) which is the statistics collection interval, then divide the result by the number of vCPUs in the VM.

So if we use the value from the “latest” column, its 7337 divide 20000, equals : 0.36685, then we divide that by 2 as the VM has 2 vCPUs and we end up with 0.183425

That’s 18% CPU Ready, which basically means 18% of the time, the VM is not doing anything!

Note: CPU Ready % can be found using ESXTOP or RESXTOP via the vMA or on the ESXi host directly.

Now to try and diagnose the Performance/CPU ready issue, we need to work out if the VM is oversized and if so, Right Size the VM.

What is an Oversized VM?

Basically a VM which has more compute resources assigned than it requires, for example, a VM which uses no more than 20% of its CPU and has 4 vCPUs.
What is Right Sizing?

In the above example, the VM is oversized as it doesn’t use more than 1vCPU (or 25%) of the CPU resources and therefore could be reduced to to 1 vCPU and run at 80%.
So the VM is oversized, and has High CPU ready, what happens when we right size it from 4vCPUs to 1vCPU and why does this help performance?
Its pretty simple, the less vCPUs a VM has, the easier job the CPU scheduler has to find enough physical cores to schedule the VM onto. If a cluster has a lot of oversized VMs, all the VMs are all competing for the same physical cores, and making it more and more difficult for the scheduler.

But what about setting a CPU Reservation? Don’t reservations “guarantee” resources?

The answer is, Yes and No.

The reservation “reserves” CPU resources measured in Mhz, but this has nothing to do with the CPU scheduler.

So setting a reservation will help improve performance for the VM you set it on, but will not “solve” CPU ready issues caused by “oversized” VMs, or by too high an overcommitment ratio of CPU resources.

In my testing I set an 80% reservation of a VMs 2 vCPUs worth of Mhz and prior to setting the reservation the CPU ready was ~20% and then CPU Ready did drop to around 10%. Note: This test was performed with only 25% overcommitment – 5 vCPUs on 4 physical Cores using CPUBUSY to keep the CPUs running at 100% (measured within the guest by Windows Task Manager).

I then set a 100% reservation of the VMs 2 vCPUs worth of Mhz, prior to setting the reservation the CPU ready was ~10% and CPU Ready did not get below 2.5% even with 100% reservation.

The result would have been exponentially worse had I tested with 50% or 100% overcommitment which is generally easily achieved with VMware and a well architected cluster. (I have seen well above these overcommitment numbers with no CPU ready issues).

Reducing CPU Ready down to 2.5% may sound like a pretty good result, but when we look at the other 3 x 1vCPU VMs on the host (4 core test ESXi 5 host) they had CPU ready of 40%!! Not to mention 2.5% is still not good!

If you have poor performance, and you discover you have High CPU Ready the best solution is  Right Size Your VMs!

I have recommended exactly that countless times and the customers never believe that performance can increase with less vCPUs, until after the Right Sizing exercise.

If after Right sizing, you still have CPU Ready, your overcommitment on CPU is simply to high for the workloads within your cluster.

You can address this by

1. Adding additional compute to the cluster. (Duh!)

2. Using Affinity rules to locate complimentary workloads together (Lots of small 1vCPU VMs which don’t have high CPU utilization will generally work well with a limited number of higher vCPU VMs)

3. Use Anti-Affinity rules to separate non complimentary workloads (eg: Don’t place all your 8vCPU VMs on one host with 300% overcommitment on CPU and expect them to work well).

4. Scaling out (not up) your VMs ie: Don’t have one 8 vCPU SQL DB server, use 4 smaller 2vCPU VMs

So now you know better than to use reservations to solve CPU contention.

Its time too go Right Sizing!

This simple task is about the best bang for buck you will get in your data center, since virtualizing on VMware in the first place.