# virtual machine reserve limit

### Question

• Hi,

I have a dual cpu quad core host environment. Total 8 CPU I see in my host task manager.

I set "0" percentage for virtual machine reserve. Let say I have this setting applied to 8 virtual machines and all of them are under the load, idle.

My question : Will each of this 8 virtual machines enjoy 16.666% CPU percentage of the Host?

I understand the math is 100/8 = 16.666 . Does this CPU figure be enjoyed by these 8 virtual machines when they have "0" percentage of virtual machine reserve percentage?

Else, can I know what is the figure? Make it simple, what is the guarantee percentage if I set all virtual machine's virtual machine reserve percentage to "0".
Wednesday, May 20, 2009 6:30 AM

• Hi,

The default in Hyper-V server is to treat all virtual machines equally. In reality, this may not be a practical or wise business decision. When allocating physical machine CPU resources to a virtual machine, it is important not to over-subscribe, i.e. trying to allocate more physical machine resources than are really available.

To help with this process, the following explanation shows the Processor configuration setting for a virtual machine.

Virtual Machine Reserve (percentage):

Percent of the logical CPU that is set aside for the running virtual machine. As each VM is started, the available capacity on the Hyper-V server itself is reduced.

Virtual Machine limit (percentage):

Percentage of logical CPU that a running virtual machine is not allowed to exceed.

Relative weight (1-10,000):

How CPU is distributed when there is contention among all running virtual machines.  The higher the number, the more processing power allocated to the VM.

Processor Functionality   Reduces vulnerability of some operating systems to high CPUID values. Unexpected, high CPUID values could cause a crash.

Best regards,

Vincent Hu

Wednesday, May 20, 2009 9:39 AM

### All replies

• Hi,

The default in Hyper-V server is to treat all virtual machines equally. In reality, this may not be a practical or wise business decision. When allocating physical machine CPU resources to a virtual machine, it is important not to over-subscribe, i.e. trying to allocate more physical machine resources than are really available.

To help with this process, the following explanation shows the Processor configuration setting for a virtual machine.

Virtual Machine Reserve (percentage):

Percent of the logical CPU that is set aside for the running virtual machine. As each VM is started, the available capacity on the Hyper-V server itself is reduced.

Virtual Machine limit (percentage):

Percentage of logical CPU that a running virtual machine is not allowed to exceed.

Relative weight (1-10,000):

How CPU is distributed when there is contention among all running virtual machines.  The higher the number, the more processing power allocated to the VM.

Processor Functionality   Reduces vulnerability of some operating systems to high CPUID values. Unexpected, high CPUID values could cause a crash.

Best regards,

Vincent Hu

Wednesday, May 20, 2009 9:39 AM
• Hi Vincent,

Thank you for responding to me.

Do you mean Hyper-V system will treat all virtual machines equally even I set the Virtual Machine Reserve limit to "0"?

If your reply is "Yes",  I power on all my virtual machines. The available capacity on the Hyper-V server (Host server) CPU limit itself is reducing. How can I monitor to know the reducing amount?

I have been researching on Perfmon section. However, I wish to know how to get an exact remaining amount from Host system.

Wednesday, May 20, 2009 1:27 PM
• Hello All,

Any update?
Friday, May 22, 2009 1:05 PM
• Monday, May 25, 2009 7:37 AM