Speed and power have long been the most important criteria when judging a supercomputer's worth as a number-crunching lab workhorse, but energy efficiency is fast catching up. The greenest supercomputers are those that can process the most scientific calculations per second while drawing the least power.

During the first half of this year, the power of the world's fastest supercomputers increased in aggregate 15 percent, whereas average supercomputer energy efficiency rose 10 percent, according to the Green500 List, a project led by Virginia Tech computer scientists Wu-chun Feng and Kirk Cameron to rank supercomputer energy efficiency. "While the supercomputers on the Green500 are collectively consuming more power, they are using the power more efficiently than before," the organization said in a statement following the release of its fifth and latest ranking in June.

The Green500 selects its members from a list of the fastest supercomputers known as the Top500, compiled by researchers at Germany's University of Mannheim, the University of Tennessee at Knoxville, and Lawrence Berkeley National Laboratory).

The greenest supercomputer on the planet is a cluster of IBM servers that helps researchers at the Interdisciplinary Center for Mathematical and Computational Modeling (ICM) at Poland's University of Warsaw High Performance Computing Center with high-end computer-intensive tasks including computer visualization, medical imaging, astrophysics, bioinformatics, fluid flow simulations and weather forecasting. ICM's supercomputer can perform more than 536 million scientific calculations per second (also known as floating point operations per second, or flops) per 1 watt of energy it uses.

The U.S. Department of Energy National Nuclear Security Administration's Los Alamos National Laboratory in New Mexico exemplifies power and efficiency with two different supercomputers taking the second and fourth slots on the Green500 list. Los Alamos's "Cerillos" supercomputer ranked second at more than 458 million flops per watt, while the lab's "Roadrunner," the world's fastest supercomputer for the past two years, came in fourth at nearly 445 million flops per watt.

Rounding out the Green500 list are IBM's Poughkeepsie Benchmarking Center, which tied for second place with 458.33 megaflops per watt; Japan's National Astronomical Observator in fifth with 428.91 megaflops per watt; and the ASTRON/University Groningen, IBM Rochester, IBM Thomas J. Watson Research Center, Max-Planck-Gesellschaft MPI/IPP and the Bulgarian State Agency for Information Technology and Communications all tied with 371.67 megaflops per watt.

Supercomputing performance is often equated with scientific prowess. In a speech last week, Russian President Dmitry Medvedev criticized his country's information technology industry for failing to develop supercomputing technology, and urged a dramatic change in Russia's use of high-performance computing, ComputerWorld reported. He pointed out to attendees of the Security Council Meeting on Supercomputers in Moscow that 476 out of the 500 supercomputers on the Top500 list were manufactured in the U.S.

Images Roadrunner and Blue Gene/P courtesy of International Business Machines Corporation. Unauthorized use not permitted.