Performance Issue with W2008 R2 Hyper-V on Intel CPUs

This KB article (KB2517329) for Windows Server 2008 R2 (including SP1) hosts with Intel Westmere or Sandy Bridge processors just popped up in my feeds.

“Consider the following scenario:

  • You have a Windows Server 2008 R2-based computer that has a large amount of physical memory and that has Intel Westmere or Sandy Bridge processors.
    For example, you have a computer that has Intel Xeon 5600 series processors and that has 48 gigabytes (GB) physical memory.
  • You install the Hyper-V role on the computer.

In this scenario, the performance of the computer may decrease.
For example, the following performance issues may be encountered:

  • The CPU usage is high and the server responds slowly when you copy large files on the computer. For example, you copy a 10-GB file.
  • The disk I/O performance of the virtual machines (VMs) is slow.
  • Windows takes a long time to start.

This issue occurs because the hypervisor supports only eight variable range Memory Type Range Registers (MTRRs). Additionally, the hypervisor cannot access the additional variable MTRRs that are introduced on recent Intel processors. Therefore, some regions of system memory are set to the default Uncacheable memory type, and the performance of the computer significantly decreases.


  • MTRRs are processor model specific registers (MSRs) that control the default caching for ranges of physical memory.
  • Intel Westmere and Sandy Bridge processors introduce additional variable MTRRs to enable systems to use a large amount of memory”.

There is a link to a hotfix on the page.  If it is applicable test it and then deploy.  Then I’d recommend (assuming your tests results are OK) making it a part of your standard host build.

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