Hyper-Threading Adds Logical CPUs

I have a SQL Server 2000 Enterprise Edition instance installed on a server with a single Intel 2.8GHz Zeon processor running Windows 2000 Server. The Processor tab in Enterprise Manager's Server Properties dialog box says that I have two processors. I'm sure the machine has only one processor. What's up?

Your server is probably using a new feature of certain Intel processors called Hyper-Threading. I'm not much of a hardware geek, so this confused me as well the first time I noticed my OS reporting more processors than the machine had. Essentially, Intel's Hyper-Threading makes one processor look like two to the OS. Each logical processor shares the same execution resources of the core processor, so you don't get the same performance benefit as if you had two actual processors. You can find more information about Hyper-Threading at http://www.microsoft.com/windows2000/server/evaluation/performance/reports/hyperthread.asp and http://www.intel.com/technology/hyperthread/.

Note that you don't have to pay for the logical processors created by Hyper-Threading technology. For example, if you have a four-CPU machine, you'd pay for four processors even if eight logical processors were available because of Hyper-Threading. However, Win2K won't use all eight processors. In this case, Win2K is incapable of differentiating between physical and logical processors. Win2K knows that your machine is licensed for four processors and won't use more than four, but it will use all four logical processors if you have two physical processors and your machine is licensed for four. In contrast, Windows Server 2003 will use all eight logical processors if your machine is licensed for four physical processors and you have Hyper-Threading enabled.

How does enabling this feature affect performance in a SQL Server environment? I haven't seen many benchmarks that evaluate the performance of Hyper-Threading, so I can't provide specific guidance. But I'm researching the effects of Hyper-Threading on SQL Server performance and will revisit this topic in an upcoming column.

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