Monday, 8 May 2017

Testing new vSphere 6.5 feature - DRS CPU overcommitment

I am currently working on a project where one of the customer's requirements is to use strict pCPU to vCPU ratio. Luckily, VMware introduced new feature called CPU over-commitment ratio in vSphere 6.5 which helps to meet the requirement. I spent an evening playing with this new feature and would like to share my experience. 

The VMware documentation is quite laconic when it discusses new DRS features. So, after reading the documentation I still had few questions on how CPU over-commitments works:

  1. Does it count vCPUs against Physical or Logical Processors?
  2. What is DRS behaviour when the ratio is violated?
  3. Is over-commitment ratio applied per host or per cluster?
  4. Will HA respect this ratio when restarting VMs after the host failure?
  5. Is ratio changed when host is placed into maintenance mode?

So, let's try to answer all these questions using my lab.

1. Does it count vCPUs against Physical or Logical Processors?

Usually I run most of my tests in the nested labs using nested ESXi servers, but to answer this question I had to use one of my physical clusters which supports hyperthreading and thus provides physical and logical processors.

The cluster consists of 2 x SuperMicro Servers and each of the servers runs on Xeon D-1528 CPU with 6 physical cores. So, in total I have 12 physical / 24 logical processors in the cluster.

Currently I am running 4 VMs with 11 vCPUs assigned in total. DRS is enabled and CPU overcommitment is configured to 100%. I am planning to power on a another VM with 2 vCPUs.
If DRS counts over-commitment ratio using physical CPUs it should give me some kind of warning.

Here is the result of my attempt to power-on another VM.

As you can see it actually answers the second question too.

We can tell now that DRS definitely counts only physical CPUs. Interestingly, in this case DRS behaves as HA Admission Control prohibiting VM power-on operation as it will violate CPU over-commitment ratio.

3. Is over-commitment ratio applied per host or per cluster?

To answer this question I used my nested lab. Here are quick specs of the test cluster:
  • 3 x ESXi servers
  • 2 x CPU per server
  • 3 x virtual machines configured with 2 vCPUs each
  • CPU over-commitment is set to 100%
So, I am running 6 vCPU in total on 6 CPUs in DRS cluster. Attempt to power on one more VM in this cluster will definitely fail as it will violate cluster level ratio. 

Now, I vMotioned VM-2 to ESXi-1 which brought the pCPU to vCPU over-commitment ratio on that host to 200%. As you can see this vMotion didn't fail and no warning were generated.

DRS generate recommendations every 15 minutes and soon this cluster was balanced again, but that's part of DRS functionality that existed in previous versions of vSphere 6.5.

So, we can tell that this over-commitment ratio is applied per cluster.

4. Will HA respect this ratio when restarting VMs after the host failure?

It was the most tickling question for me. Taking into the consideration similarity of CPU over-commitment and HA Admission Control features I was wondering whether over-commitment ratio should be adjusted to take into the consideration host failure.

I used the same lab setup you saw above in question 3. I verified that each host has been running one dummy VM.

Then I restarted vesxi65-3 host and 2 minutes later the VM-3 was successfully restarted on vesxi65-1 server even though the CPU over-commitment ratio was equal to 150%.

This proves that HA restart has higher priority over CPU over-commitment ratio. This totally makes sense to me as VM's availability is more important that potential performance impact.

5. Is ratio changed when host is placed into maintenance mode?

I reverted my lab back to default settings and tried to place the host into maintenance mode which would result in 4 pCPU to 6 vCPU ratio which would violate configured CPU over-commitment ratio. 
The tasks didn't fail so I at first I assumed that there would be no problem.

5 minutes later that task was still running so I checked the DRS Faults and immediately found the following.

Clearly, DRS would always respect its own over-commitment rule when trying to generate vMotion recommendations. 

So, the main takeaways for today are:

  • Only physical CPUs are used in calculations - no hyper threading
  • CPU over-commitment works very similar to Admission Control by preventing VMs to power on if it will violate the configured ratio.
  • During HA failover the CPU overcommitment setting is ignored - makes sense as recovering VMs is more critical than respecting overcommitment ratio
  • The over-commitment ratio is applied at cluster level
  • DRS will prevent placing the host into maintenance mode if it breaks its rules. 


  1. Really useful to know thanks

  2. Thanks for the awesome write up ! Very useful..

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  4. Thanks a lot for sharing your findings.
    A couple of things however:
    1) 100% over-commitment supposedly means exceeding the cluster capacity by 100%. In other words, trying to commit twice as much resources as actually available.
    2) It doesn't look like you tested the hyper-threading aspect as the screenshot of your physical cluster shows 12 Logical Processors, i.e. the same as physical CPU cores in the cluster.
    Can you please shed more light on the points above ? Am I missing anything here ?