The recently released Green500 ranking takes the list of the world's 500 fastest supercomputers and re-orders them based on efficiency, as measured by performance per watt. No company dominates this metric quite like IBM.

Big Blue's Roadrunner system at the Department of Energy's Los Alamos National Laboratory has maintained the lead spot in the Top 500 Supercomputer Sites, which ranks machines by raw speed. But Roadrunner came in fourth place in the list, behind three other IBM installations.

The world's most efficient is an IBM BladeCenter cluster at the University of Warsaw, which produces 536 MFlops for each watt of energy used. An MFlop is equal to 1 million floating point operations per second. In terms of raw speed, the Warsaw computer is the 422nd fastest.

Two IBM computers tied for second place in the Green500 list: another machine at the Department of Energy and one at IBM's Poughkeepsie Benchmarking Center. The first non-IBM machine clocked in at No. 5, that being the Greatly Reduced Array of Processor Elements with Data Reduction (GRAPE-DR) system at the National Astronomical Observatory of Japan, which produced 429 Mflops per watt.