IBM has lost the supercomputer crown to Compaq after holding the top slot for the last three years.

The new chart topper is a 3,024-processor Compaq machine called Terascale based at the Pittsburgh Supercomputing Centre.

The Terascale can perform six trillion calculations per second, the equivalent of 10,000 desktop PCs.

It relegated IBM's Asci White into second place following a chart shake-up based on a more sophisticated way of ranking of supercomputers.

Experts say the new list compiled by research firm IDC better represents real-world performance.

"The excessive focus on peak performance has contributed to a tendency for high-performance computing vendors to develop new generations of supercomputers showing impressive gains in peak performance while lagging behind in many critical system features," said IDC analyst Debra Goldfarb.

Measuring performance

Many were dissatisfied with the way performance had been measured in the past.

Previous lists were based on the Linpack measurement, which measures how well processors work but excludes other factors, such as how fast data can be transferred from one part of the system to another.

Experts were concerned that this was misleading, as it reflected peak performance, rather than how well the supercomputer worked in practice.

The new way of rating the performance of supercomputers includes the Linpack measurement. But it also measures several other elements:

• The processor's mathematical abilities
• How fast the computer can transfer data to memory
• The total number of processors combined with the memory transfer capacity

The new way of measuring the power of supercomputers is designed to help companies make better decisions when they consider which high-performance machine to purchase.

Unlike the traditional supercomputer systems, today's largest high-performance computers are built by coupling together many thousands of standard processors.

The Terascale system uses technology developed in the UK to connect together its 3000 Compaq Alpha processors.

Quadrics, based in Bristol, designed the specialised switches which allow all the processors to talk to each other, as well as software that allows all the processors to co-operate on a single task.

As supercomputer systems continue to grow in size and power, the means to accommodate even larger processor counts will become increasingly critical.