Hyper-Threading in Windows Server 2003. (Software Intelligence).

Butler Group

The first release dates of Windows Server 2003 will see launch promotions lauding many new features in the eagerly awaited Windows 2000 successor, in particular the integration of .NET in the server. What the new server will also realise is the full potential of a technology that Intel has built into the current generation of Pentium 4 processors since last year, namely Hyper-Threading (HT).

HT allows a single physical processor to he accessed as two virtual processors by executing two separate software threads (or instruction streams) simultaneously. The virtual architectural states appear as two processors to the Operating System (OS), which sees only the virtual instances when running applications. In multitasking applications the technology switches from one set of data instructions to the other, every few nanoseconds, keeping track of each thread's status.

However, the virtual processors must share common execution resources on the single physical processor, so performance gains are less than 100%; more of the order of up to 30%. These performance gains and improved system responsiveness are realised when running multiple applications in parallel.

This technology is only available on Intel Pentium chips bearing the logo 'Intel Pentium 4 Processor with NT Technology', that run at 3GHZ or higher and have the appropriate Intel specification BIOS-this is important since Windows receives processor information from the BIOS. In a multi-processor server, such a BIOS will label the first logical processor in each physical processor in numerical sequence, and then loop back to the first physical processor and assign a processor count label to the second logical processors m continuing sequence.

With the technology in place, the OS and application must support it for the features to be enabled. While this is possible in Windows 2000, a limitation in not distinguishing between physical and logical processors causes the logical processor to be counted against licence CPU limits. For example, on Windows 2000 Server with 4-CPU licence limit and Intel specification BIOS on a four-way system, the OS will use the first logical processor on each of the four physical processors. This makes redundant the second logical processor on each processor due to the licence limit. The situation is changed in Windows Server 2003, which can now make a distinction between physical and logical processors, and which takes the crucial processor licence count against the physical processor. Using the same example, of a 4-CPU licence limit on a four-way server, the system can now make use of eight logical processors, with a signilficant performance enhancement for the same licence fee.

Butler Group Opinion:- NT is a technology that boosts multi-tasking performance on Windows servers running on Intel processors. Whilst multi-tasking achieved by software alone has been a feature of Windows since version 3.1, the HT hard ware support provides significant enhancements. Certainly the new Windows Server 2003 provider a cost benefit to the user on the HT feature alone.

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