Tests of the latest Intel Xeon workstations indicate that Intel's HyperThreading feature, designed to speed up some types of processing, can degrade the performance of compute-intensive applications by up to 25 percent.

Companies should not assume therefore that the Xeons' HyperThreading feature will always have a benign or beneficial effect.

Currently HyperThreading is used only in Intel's Xeon DP and MP chips, but it is likely to appear in 64bit Itanium chips by 2005. Similar features could also be added to rival processors such as Sun's UltraSparc.

According to Intel, HyperThreading can boost system performance by about 30 percent. Strategic marketing manager Alan Priestley said, "Some applications will benefit more than others. Some applications run better with HyperThreading disabled because of the way they are written. We recommend hardware vendors ship workstations with HyperThreading disabled."

Server-based software is likely to gain most from HyperThreading. In contrast, applications that repeatedly use a particular portion of the Xeon chip's resources, such as its floating-point features, may see performance fall with HyperThreading enabled.

This issue has been understood for some time, but the IT Week Labs results are the first to show the severity of the degradation. IT Week Labs ran computational fluid dynamics software from specialist vendor Fluent on a Dell workstation fitted with two 2.2GHz Intel Xeon DP processors, running Suse Linux Enterprise Server. Fluent's FL5L2 test processes about 2GB of data using floating-point calculations to simulate wind-tunnel testing and is one of a set of common benchmark tests for high-end workstations.

Normally the results scale up proportionately as more chips are added, and Intel's descriptions of HyperThreading may lead some customers to expect quad-processor levels of performance from a dual-CPU system. However, the simulation took 25 percent longer to run with HyperThreading enabled.

Fluent said that it has not run any benchmarks on systems using HyperThreading and that its software has not been optimised for this feature. Fluent said it is working on a version for the Itanium 2 chip, but has no plans to support HyperThreading. Fluent's Web site lists benchmarks for a variety of processors, including IBM's Power4, HP's Alpha and Intel's Xeon.

The best result we recorded was equal to results published by Fluent for workstations configured with similar hardware.

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