Dubbed the Jaguar, the mission-critical Cray XT5 Linux supercomputer system is based at Oak Ridge National Laboratory. It was recently upgraded from quad-core to six-core AMD Opteron processors.
With the wind behind it and going downhill the Jaguar manages 2.3 petaFLOPS theoretical peak performance and 1.75 petaFLOPS performance if you use the Linpack benchmark.
AMD is doing quite well in the supercomputer league tables with four of the five top systems running on its chips.
Number two on the list was IBM's Roadrunner which first appeared in June 2008 and was the first computer to surpass 1 petaFLOPS on the Linpack benchmark test.
It still held the top spot in June 2009 with 1.105 petaFLOPS, but lost its place after being repartitioned.