For a variety of reasons -- access to more RAM, better performance, etc. -- many people have chosen to use Windows Server 2008 as a desktop solution. With Hyper-V -- one of the newest products to arrive on the enterprise virtualization scene -- is a free add-on for this server product that might be a logical choice to use for virtual testing. However, Hyper-V and higher-end video cards don't get along. Scott Lowe, with a little help from the Microsoft Knowledge Base, explains why.
Partly due to perceived shortcomings in Vista and party due to need, there has been a great deal of interest in running Windows Server 2008 as a workstation. Personally, I run the 64-bit edition of Windows Server 2008 on both my home and work machines. The additional RAM possibilities and the ability to use some Windows Server 2008 features make this a desirable configuration.
If you do any amount of testing, installing the Hyper-V role might be on your to-do list. Although there are a whole lot of options out there when it comes to virtualization, at first glance, adding the Hyper-V role might seem to be the easiest path to take.
If your desktop is doubling as your lab environment, you might want to reconsider this choice and pick a different virtualization option.
A couple of days ago, I installed the Hyper-V role on my work Windows Server 2008 desktop. The installation of the Hyper-V role requires a reboot. Immediately upon reboot, my system slowed to an absolute crawl. It was so bad that it was almost to a point of being unusable. The mouse pointer was choppy, application windows redrew slowly, if at all. This was a weekend activity on my part, so I was also watching a Flash-based video on Hulu while I worked; the video performance was so bad that it wasn't watchable. Of course, that alone wasn't a major problem, but the system slowing to an overall crawl was a big problem.
After I uninstalled the Hyper-V role, system performance returned to normal.
I was baffled and decided to do a little research into this weirdness. I ran across a Microsoft knowledge base article that explains the reason for this behavior:
"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."
In other words, common tasks that use certain video functions require an inordinate amount of overhead when the Translation Lookaside Buffer is virtualized. As a result, system performance is seriously impacted. I'll admit that a lot of what is explained above is relative gibberish, but the last couple of sentences make it clear that video drivers are the culprit.
Microsoft's suggestion: Roll back to the stock Vga.sys or Vgapnp.sys generic video drivers that are included with Windows Server 2008; uninstall your high-end video drivers. The generic drivers do not use the methods that create performance-degrading conditions and keep your system stable and responsive. Of course, by not using the high performance drivers, a lot of other things won't work and the system isn't of much use as a decent desktop.
My recommendation: If your Windows Server 2008 system has a good graphics adapter and splits its time between desktop and virtual lab duties, don't use Hyper-V. Instead, choose to use Virtual PC 2007, VMware Workstation or Server, Sun VirtualBox or Parallels.
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