News

Technical Discussion of the Xeon MP Throttling

• By Stephen Swoyer
• 03/13/2002

But the system tested in the benchmark fits into a narrow loophole of scenarios that can properly exploit Intel's new Hyper-Threading technology, a major scalability feature of the Intel Xeon MP processor that was launched Tuesday.

A Microsoft Corp. technical white paper, however, explains that Windows 2000 Server in many cases cannot properly exploit the Hyper-Threading technology in the Xeon MP.

Hyper-Threading allows a single physical processor to act as two logical processors by executing multiple threads simultaneously.

The problem arises from the interaction of the way Windows 2000 Servers interpret BIOS information at startup and the way Intel Xeon MP's Hyper-Threading technology presents itself to the system.

The Intel Xeon MPs implement Hyper-Threading by presenting a virtual processor to the BIOS in addition to the physical processor. According to the system, then, each physical processor presents two logical processors.

When a Windows 2000 Server runs on OEM hardware with a system BIOS meeting Intel's specifications, the operating system will fill out its license limit using the first logical processor on each discrete physical Xeon microprocessor.

“Windows 2000 Server does not distinguish between physical and logical processors on systems enabled with Hyper-Threading Technology; Windows 2000 simply fills out the license limit using the first processors counted by the BIOS,” Microsoft’s whitepaper says.

On a standard four-physical-processor server, the system BIOS must first cycle through logical processors 1-4 on each of its four discrete microprocessors before repeating the process to count logical processors 5-8.

On a Windows 2000 Server system with a four CPU limit running on four-way Hyper-Threaded hardware, then, only the first four logical processors will be used by the OS.

On the other hand, a Windows 2000 Advanced Server system with an eight CPU limit installed on the same hardware will run across all eight logical processors. In this configuration, Windows 2000 Advanced Server will treat logical processors 5-8 as physically separate microprocessors. This was the scenario in the Intel benchmark.

Microsoft also notes that on hardware that doesn’t conform to Intel’s BIOS specifications, Windows 2000 Server could run across only the first two processors in a four-way SMP configuration. That’s because if a system BIOS doesn’t count logical processors 1-4 on each discrete microprocessor before starting over at the beginning to tabulate logical processors 5-8, Windows 2000 Server could load across only the first two physically separate microprocessors (logical processors 1,2,5,6). In this configuration, then, two of the system’s four physically separate microprocessors could remain untapped.

To be sure, Intel's choice of a four-processor Windows 2000 Advanced Server system to run the mySAP Supply Chain Management benchmark publicized at the launch on Tuesday is legitimate. Four-processor systems make up the majority of demand for the new Intel Xeon MP processors, and Intel is legitimately interested in demonstrating the scalability of those systems using the new processor.

But it is clear from the Microsoft technical white paper that whatever portion of the performance gain Intel's test system got from the Hyper-Threading technology would not be replicated under many other common Windows 2000 configurations.

Here is how the issue works out in various combinations of operating systems and physical processors: