Maximum CPU capacity is less than expected in Virtual Server 2005 (867586)

SYMPTOMS

When you view resource allocation in Microsoft Virtual Server 2005, Virtual Server 2005 reports the maximum capacity percentage of the virtual machine. If you use hyperthreaded CPUs, the percentage might be less than you expect.

CAUSE

This behavior occurs when you use hyperthreaded CPUs. For example, when you have four physical hyperthreaded Intel Xeon processors, the maximum CPU allocation per virtual machine is 12.5 percent. This value is determined by dividing the total maximum capacity (100 percent) by the eight virtual processors. There are two virtual processors for each physical processor in a hyperthreaded CPU. Virtual Server 2005 can assign only one CPU to each virtual machine.

MORE INFORMATION

Hyperthreaded CPUs contain a second (virtual) CPU. With this feature, multithreaded applications can run threads in parallel in each processor. When you use Virtual Server 2005, you may want to disable hyperthreading. In certain cases, if hyperthreading is enabled, virtual machines that are running on the server may experience decreased performance.

For additional information about Microsoft Windows support for hyperthreading, visit the following Microsoft Web site:

http://www.microsoft.com/whdc/system/CEC/HT-Windows.mspx

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