The Green 500 list compiled by researchers at Virginia Tech is intended to draw attention to idea that designers of supercomputers should pay more attention to the energy consumption, not just speed or computational prowess.

"In the world of supercomputers, the thinking has traditionally focused entirely on performance," says Virginia Tech associate professor Wu Feng, who spearheaded the Green 500 project with Kirk Cameron. "No one has worried about the power being consumed. Well, the world has changed. There is a lot of concern right now about the amount of power being consumed by computers and data centers in general, and we felt it was time to do something that really challenged the thinking of the (supercomputer) establishment."

In fact, as manufacturers have pursued the goal of building supercomputers that can complete hundreds of trillions of floating-point operations per second they have inadvertently created computers that consume so much energy and produce so much heat they require elaborate cooling systems to ensure their proper operation.

The first Green 500 list will be refined in the months and years ahead, adds Feng. Initially, not all companies with computers on the Supercomputer 500 ranking would or could provide energy consumption metrics for their machines. However, based on the feedback received to date, and the attention the Green 500 is generating, Feng believes the list will be more comprehensive in the future and the methodology more refined.

The top of 2007's list is dominated by IBM's Blue Gene supercomputers. In fact, IBM had nine out of 10 of the top 10 sites. The only non-IBM installation to crack the top 10 was a Dell PowerEdge cluster at Stanford University. In terms of flops per watt, the top installation was an IBM Blue Gene supercomputer installed at the Science and Technology Facilities Council Daresbury Laboratory in Cheshire, England. It achieves 357.23 megaflops per watt.

Much of IBM's success is due to the use of more efficient processors. The new generation of Blue Gene supercomputers use 850 MHz CPUs compared to 2GHz CPUs used in most supercomputers.

While this is the Green 500's first year, the genesis for the list dates back to 2001 when Feng was working at the Los Alamos National Laboratory. He was struggling to maintain the reliability of a supercomputer installed at the laboratory, which was overheating due to the high number of power-hungry processors.

That led him to design a new supercomputer, which focused as much on energy efficiency as power. The result was a machine named Green Destiny, which used 240 Transmeta processors, operating at 667 MHz, and sipped only 3.2 kilowatts of power (about the same as two hair dryers).