"It will be quite exciting if they are able to hit that mark, but it's still unclear if they can do it," said Jack Dongarra, a professor at the University of Tennessee who helps manage the Top 500 list of the world's most powerful computers.

The Top 500 list is published in June and November each year around major supercomputing conferences. Typically April 15 is the cut off date for the June rankings, but list administrators granted an extension to the Los Alamos team as it races to characterize the performance of its Road Runner system.

"We have made a major exception for them until May 15 and can provide a few more days beyond that," said Dongarra. "It's a big deal if they can do it, so we want to give them every possibility."

A petaflops is a quadrillion floating point operations per second. Flops are used as a yard stick to measure typically scientific systems that tend to push the limits of computing.

The Top 500 managers provide supercomputer centers with a set of problems they need to run on their machines. They review output from the supercomputers to determine the performance of tested machines which they then rank.

Dongarra described the Road Runner system as "quite powerful and unique." It uses a mix of AMD dual-core Opteron processors as well as an IBM version of the Cell processor, originally developed by IBM, Sony and Toshiba for the PlayStation.

The Cell processor itself includes a relatively conventional IBM Power core as a control processor as well as eight specialized vector processors which the Cell designers call Synergistic Processing Elements. "That adds to the overall complexity," Dongarra said.

Several other systems are actively engaged in a race to break the petaflops barrier, but none are in a stage of actually testing performance for the June rankings. They include a supercomputer at the University of Texas in Austin built by Sun Microsystems, an Intel Xeon system at NASA and a Cray XP4 system also using AMD Opteron chips being installed at Oak Ridge National Lab.

Dongarra said any of those machines might be in a position to get tested in time for the November rankings.

Japan had the world's most powerful supercomputer, the Earth Simulator for five iterations of the Top 500 list starting in 2002. But in November 2004, IBM's 70 TFlops BlueGene/L system at Lawrence Livermore National Lab leapfrogged the 35 TFlop Earth Simulator. Since that time the BlueGene system has remained the most powerful system in the world and is now rated at about 478 TFlops.

Japan has announced a follow on project called the Life Simulator targeted at achieving 10 petaflops of sustained performance. But it is not expected to be ready until 2011, Dongarra said.

Japan has at least three other systems approaching the petaflop mark, but they will probably not be available for testing until 2009, Dongarra said. Efforts in Europe and China are further behind, he added.