Japan's K Computer has retained its pole position on the Top500 list of fastest supercomputers and become the first machine on the list to achieve performance of more than 10 petaflops.

The computer, developed by Fujitsu and the Riken science institute in Kobe, Japan, clocked in at 10.51 petaflops or more than 10 quadrillion calculations per second on the Linpack benchmark test, the list's organisers announced.

That was four times the speed of its nearest competitor, China's Tianhe-1A system, which achieved 2.57 petaflops, powered by Intel Xeon and Nvidia graphics chips. The fastest US machine, a Cray XT5-HE computer at the Oak Ridge National Laboratory, was in third place at 1.75 petaflops. It uses AMD Opteron processors.

The first system to achieve 1 petaflop was IBM's Roadrunner in June 2008, so it took a little more than three years to improve on that by tenfold. At the time, Top500 editor Erich Strohmaier said he had never expected to see a petaflop-class machine when he helped start the list in 1993.

"No, 15 years ago the big question was whether all 500 systems together would amount to 1 teraflop," he said.

The Japanese machine is housed in 864 server racks and has 705,024 Sparc64 VIIIfx processor cores. That's up from 548,352 cores when it made its debut at the top of the list in June, when it was measured at 8.16 petaflops. At the time it wasn't fully built out.

Software is still being developed for the K, and it will officially go into operation late next year, for work such as weather modelling, drug development and analysis of nanotubes and nanowires for use in tomorrow's semiconductor chips.

The K system draws 12.66 megawatts of power, or enough for about 12,000 homes, but it's also one of the most energy efficient systems on the list because of the amount of work it gets done with that power. It was expected to do well after Fujitsu reported its unaudited Linpack result last week.

China and Japan each had two systems in the top 10, while the US had five and France one. That was the same as in June, since the same 10 machines occupied the top 10 spots, just in a different order. It's the first time that's happened since the list was started.

China, in particular, has been improving its position. It had 75 systems in the top 500 and "is clearly the No. 2 country" as a user of high performance computing, ahead of Japan, the UK, France and Germany, the list's editors said.

More than three quarters of the systems run on Intel processors, which is down a fraction from last time. Sixty three systems use AMD Opterons and 49 use IBM's Power chips.

The K Computer doesn't use graphics processors as accelerators, but 39 systems on the list do, up from 17 in June, with the vast majority using Nvidia chips.