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Core Speed on VMware vCPU Lower on one VM

I have thee VMs that have the same OS installed and are allocated the same resources CPU, Memory, etc.  All three VMs are located on the same host.

The core speed on the host is 1.17 GHz.  I have 4 vCPUs assigned to each VM.

One of the three VMs is only showing 1.06 GHz, instead of 2.27 GHz like the other three.  What could cause this?
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1 Solution
Nagendra Pratap SinghCommented:
Use one vCPU for all machines. That is the best practice ( For high performance).

reboot and then see again.  Is it VMware workstation?
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Can you provide a screenshot of this?
LarsArvidsonAuthor Commented:

Why would I limit the vCPU on these VMs when the (multi-threaded) application can max out 4?  Also, this doesn't explain why the VM OS doesn't recognize the core speed.  I'm running vSphere 5.1.
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
The Virtual Machine OS will report the physical details of the Host CPU to the virtual machine OS.

e.g. I have a Dell 2950 with Xeon L5335, 2.0GHz processors, (they do have multiple cores).

When I allocate 1 vCPU (which in layman terms equals 1 physical core on the physical processor, for a moment in time, before it's shifted to another core - possibly!).

The VM OS will show 2.0GHz.

if I allocate 2 vCPUs it does not show 4.0GHz in the OS.

Can you upload a screenshot?
LarsArvidsonAuthor Commented:

Here are the screenshot's.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
thanks for screenshots....
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
that looks like CPU contention.

or there is a CPU reservation setting, or limit, or Resource Pool, clamping the CPU setting for the VM.
LarsArvidsonAuthor Commented:
You can see from the host summary screenshot and from the performance graph that there doesn't look to be much CPU contention.  Also, no reservations.  It doesn't look like I have any resource pools.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
is it just a single VM?

VMware Tools is installed?

and it's the same host as other VMs, not showing this issue, and this occurs all the time?
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
You can also read this article...

which makes me wonder if it's resources taken away from the VM...

"......There are other reasons that the internal measurements can differ, such as clock skew and adjustments, but suffice it to say that if you are looking for an absolute view of how CPUs are being used, you cannot trust the guest’s view of itself....."


if would be interesting if reduce to a single vCPU it returned to normal.

Also there are no powering saving functions enabled in the BIOS of your Host Server.
LarsArvidsonAuthor Commented:
I changed the VM to one CPU and the clock speed shows up correctly now.  Thanks for the help.
LarsArvidsonAuthor Commented:
Thanks for the help hanccocka!
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
no problems...

further reading if interested...

vSMP (virtual SMP) can affect virtual machine performance, when adding too many vCPUs to virtual machines that cannot use the vCPUs effectly, e.g. Servers than can use vSMP correctly :- SQL Server, Exchange Server.

This is true, many VMware Administrators, think adding lots of processors, will increase performance - wrong! (and because they can, they just go silly!). Sometimes there is confusion between cores and processors. But what we are adding is additional processors in the virtual machine.

So 4 vCPU, to the VM is a 4 Way SMP (Quad Processor Server), if you have Enterprise Plus license you can add 8, (and only if you have the correct OS License will the OS recognise them all).

If applications, can take advantage e.g. Exchange, SQL, adding additional processors, can/may increase performance.

So usual rule of thumb is try 1 vCPU, then try 2 vCPU, knock back to 1 vCPU if performance is affected. and only use vSMP if the VM can take advantage.

Example, VM with 4 vCPUs allocated!

My simple laymans explaination of the "scheduler!"

As you have assigned 4 vCPUs, to this VM, the VMware scheulder, has to wait until 4 cores are free and available, to do this, it has to pause the first cores, until the 4th is available, during this timeframe, the paused cores are not available for processes, this is my simplistic view, but bottom line is adding more vCPUs to a VM, may not give you the performance benefits you think, unless the VM, it's applications are optimised for additional vCPUs.

See here

see here


also there is a document here about the CPU scheduler

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