A monstrous number cruncher still only half complete has retained its title as the most powerful computer on the planet.

Blue Gene/L, built by IBM at Lawrence Livermore National Laboratory in the US, regained the top spot after recording a peak processing speed of 136.8 trillion calculations per second - or teraflops - in testing. Once complete, the system should to be twice as powerful, IBM says, with a peak performance in excess of 360 teraflops.

The list of the 500 most powerful computers anywhere - known as the Top 500 list - is released twice a year by researchers from Germany and the US. The latest list was announced at the International Supercomputer Conference in Heidelberg, Germany, on Wednesday.

It is topped by exotic, custom-built supercomputers, rather than those built up from standard components. But also bears testament to a rapid general rise in the power of supercomputers.

Modular design

For the past couple of years, many of the computers close to the peak of the Top 500 list have been constructed using ordinary computer hardware, linked together with off-the-shelf software. But this is changing, with customised systems starting to dominate the rankings.

The current champion, Blue Gene/L, for example, is a specialised system designed to eventually incorporate an unprecedented 65,536 individual processors, linked together within a scalable, modular design. The system is not only much more powerful than conventional systems but also more compact and less power hungry.

The strength of the design is emphasised by the fact that second on the new list is another Blue Gene machine, known as BGW and based at IBM’s Thomas J Watson Research Center in New York. This system features the same modular design as Blue Gene/L but has fewer processors and has recorded 91.2 teraflops peak processing power.

Rapid pace

Third placed on the list is another machine made from specialised components - SGI's Columbia at NASA Ames Research Center in California, which clocked up 51.9 teraflops in testing.

The new list reflects the rapid pace of change in the capabilities of supercomputers. More than half of the machines in the top 10 have been deposed since the rankings were last compiled in November 2004.

And, for the first time ever, every machine on the list is capable of a peak performance of at least teraflop. None of the computers that made the list in 1997 would now qualify for a place in the rankings.

The battle for supercomputing supremacy shows little sign of waning either, as the Japanese government recently announced plans to fund development of a machine capable of 1000 teraflops - or a petaflop - by 2010.