The latest supercomputer horse race came down to a photo finish.

IBM's Roadrunner inched out Cray's newly beefed-up XT5 Jaguar to retain the top spot on the bi-annual Top500 list of supercomputers. The latest list was released Monday.

Roadrunner, which is installed at Los Alamos National Laboratory, had its own upgrade in the past few months, enabling it to hit 1.105 petaflops, or more than a quadrillion mathematical calculations per second. Jaguar, the second of only two computers to break the petaflop barrier, grabbed second place on the list, with a top performance of 1.059 petaflops running the Linpack benchmark application.

"That was really quite a thing," said Jack Dongarra, a co-creator of the Top500 list and a distinguished professor at the University of Tennessee. "Ultimately, both machines are incredibly capable systems."

Roadrunner is a hybrid system, running 12,960 IBM PowerXCell 8i Cell Broadband Engine processors and 6,948 AMD Opteron dual-core processors. The Opteron chips handle basic computer functions, while the Cell chips take care of the computations.

Jaguar, on the other hand, runs 45,000 Opteron quad-core chips, which adds up to a total of 180,000 processors.

IBM has been quick to note that while the two machines are so close in performance, Roadrunner uses about half the power that Jaguar does. Roadrunner reportedly uses 2.5 megawatts, compared to Jaguar's 7 megawatts of power consumption.

But Dongarra says the comparison needs to go deeper than that. He noted that Jaguar has a lot more memory than Roadrunner, which adds to Jaguar's power consumption. Also, he pointed out that programmers must code software for each the hybrid Roadrunner's different types of processors while Jaguar is a more traditional system, calling for more traditional - and easier -- parallel programming.

"Jaguar will be easier to use and easier to program by a wider group of people." said Dongarra. "Because of programmability, maintenance and support, it may be more cost efficient to use the Cray system."

Dongarra also noted that seven of the top 10 supercomputers on the list are operating at laboratories run by the Department of Energy.