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Video performance may decrease when a Windows Server 2008 or Windows Server 2008 R2 based computer has the Hyper-V role enabled and an accelerated display adapter installed

Article ID: 961661 - View products that this article applies to.

SYMPTOMS

When you run Windows Server 2008 or Windows Server 2008 R2 with the Hyper-V role enabled, video performance may decrease when you perform certain actions. For example, you may experience this issue when you press CTRL+ALT+DELETE to switch from the desktop to the logon screen or when you run certain graphics-intensive applications.

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CAUSE

This issue occurs when a device driver or other kernel mode component makes frequent memory allocations by using the PAGE_WRITECOMBINE protection flag set while the hypervisor is running. When the kernel memory manager allocates memory by using the WRITECOMBINE attribute, the kernel memory manager must flush the Translation Lookaside Buffer (TLB) and the cache for the specific page. However, when the Hyper-V role is enabled, the TLB is virtualized by the hypervisor. Therefore, every TLB flush sends an intercept into the hypervisor. This intercept instructs the hypervisor to flush the virtual TLB. This is an expensive operation that introduces a fixed overhead cost to virtualization. Usually, this is an infrequent event in supported virtualization scenarios. However, some video graphics drivers may cause this operation to occur very frequently during certain operations. This significantly magnifies the overhead in the hypervisor.

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RESOLUTION

When you enable the Hyper-V role in Windows Server 2008 or in Windows Server 2008 R2, do not install the drivers for high performance accelerated graphics adapters. This behavior will not occur when you use the Vga.sys or Vgapnp.sys generic video drivers that are included with Windows Server 2008 and with Windows Server 2008 R2. To revert to the generic video driver, you can uninstall any high performance vendor-specific video driver.

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