Q. I understand that the number of virtual processors per logical processor supported with Windows Server 2008 R2 SP1 has changed. What's the new ratio?

A. First, it's important to note that the support is virtual processors to logical processors, not physical cores. This is vital, because many of today's CPUs have features such as hyperthreading, which makes one physical core look like two logical processors. So when Microsoft says the ratio is based on logical cores, it means your quad-core processor with hyperthreading counts as eight logical processors.

With Server 2008 R2 SP1 running only Windows 7 client OSs in your virtual machines, the new supported number of virtual processors for each logical processor is 12 (a ratio of 12:1). If you have any guests that aren't running Windows 7, the supported ratio remains 8:1.

This is good news for Windows 7 Virtual Desktop Infrastructure environments. That single quad-core processor with hyperthreading now supports 96 single virtual processors for Windows 7 guests! The new six-core processors would be 144, and if you put multiple processors in a single box, you can start to see the difference this 50 percent increase in virtual processors to logical processors makes.

See this Microsoft page for details about the supported limits for Hyper-V.