Intel's quandary over hyper-threading future

Hyper-threading is something of a mystery, and Intel are still unsure how to release it successfully to the industry.

Every Pentium 4, and thus Xeon - which has the same basic architecture, has nine execution units inside (the bits of the chip that actually do sums). Although the Pentium goes to great lengths to keep those busy by carrying out multiple instructions at once, it can only cope with one lot of programming code shuttling through its vitals. This is not enough: on average, only 35% of the chip is actually working out problems at any one time.

Hyper-threading adds extra circuitry that copes with a second stream of program code and data (a thread) simultaneously. To the operating system and applications, it looks as if there are two completely independent processors on the motherboard, and if the software is designed to make use of this then the processor will use a lot more of its capabilities at the same time.

The reason the feature has not been turned on yet in Pentium 4's is that if you turn the feature on, software that does not need multiple threads (all current software) will go slower.

However, Open magazine tested Intel's latest C/C++ compiler, the piece of software that turns programmers' code into executable software, and the magazine's software performance improved by between 30% and 50%.