KB2885465 – CPU Not Allocated Correctly VMs On Win8 Or WS2012 Hyper-V

Microsoft has released a KB article for when CPU resources are not allocated correctly for a virtual machine running on Windows 8 Client Hyper-V or Windows Server 2012 Hyper-V.

This article is related to unexpected behaviour with the virtual processor resource control settings of a VM in Hyper-V.  Most people never touch these settings, and probably aren’t even aware of what they do.  My guess is the only people who touch them are maybe hosting companies, and those who want to dedicate processor to SQL Server, Exchange, or SharePoint VMs by reserving 50% or 100% of a logical processor (physical core, or half core with Hyperthreading enabled) capacity to each vCPU in the VM.  That’s probably why this article has appeared now rather than a long time ago.

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Symptoms

When the Hyper-V role is installed on a Windows Server 2012-based computer, or the Hyper-V feature is enabled on a Windows 8-based computer, you experience the following issues.

Issue 1

When you set the CPU limit on a virtual machine to a value that is 15 percent or less of the total CPU resources on the computer, the virtual machine crashes.
Note This issue does not occur when the virtual machine is running Windows 8, Windows Server 2012, or a later version of Windows.

Issue 2

When you configure the CPU limit on a virtual machine, the virtual machine is allocated less resources than the limit that you configured. For example, if you set the CPU limit on a virtual machine to 20 percent of CPU resources on the computer, the virtual machine is allocated less than 20 percent of CPU resources.

 

Cause

Cause of Issue 1

This issue occurs because the timer clock interrupts are not sent to the virtual machine in time. Therefore, the virtual machine assumes that a hardware error occurred.

Cause of Issue 2

This issue occurs because the hypervisor throttles the resources that are provided to the virtual machine.

A supported hotfix is available from Microsoft.

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