Computer science. What it'll take to go exascale.

R E D IT : J . C H E N /C E N T E R F O R E X A S C A L E S IM U L A T IO N O F C O M B U S T IO N I N T U R B U L E N C E , S A N D IA N A T IO N A L L A B O R A T O R IE S USING REAL CLIMATE DATA, SCIENTISTS AT LAWRENCE BERKELEY National Laboratory (LBNL) in California recently ran a simulation on one of the world’s most powerful supercomputers that replicated the number of tropical storms and hurricanes that had occurred over the past 30 years. Its accuracy was a landmark for computer modeling of global climate. But Michael Wehner and his LBNL colleagues have their eyes on a much bigger prize: understanding whether an increase in cloud cover from rising temperatures would retard climate change by refl ecting more light back into space, or accelerate it by trapping additional heat close to Earth. To succeed, Wehner must be able to model individual cloud systems on a global scale. To do that, he will need supercomputers more powerful than any yet designed. These so-called exascale computers would be capable of carrying out 10 fl oating point operations per second, or an exafl op. That’s nearly 100 times more powerful than today’s biggest supercomputer, Japan’s “K Computer,” which achieves 11.3 petafl ops (10 fl ops) (see graph), and 1000 times faster than the Hopper supercomputer used by Wehner and his colleagues. The United States now appears poised to reach for the exascale, as do China, Japan, Russia, India, and the European Union. It won’t be easy. Advances in supercomputers have come at a steady pace over the past 20 years, enabled by the continual improvement in computer chip manufacturing. But this evolutionary approach won’t cut it in getting to the exascale. Instead, computer scientists must fi rst fi gure out ways to make future machines far more energy effi cient and tolerant of errors, and fi nd novel ways to program them. “The step we are about to take to exascale computing will be very, very diffi cult,” says Robert Rosner, a physicist at the University of Chicago in Illinois, who chaired a recent Department of Energy (DOE) committee charged with exploring whether exascale computers would be achievable. Charles Shank, a former director of LBNL who recently headed a separate panel collecting widespread views on what it would take to build an exascale machine, agrees. “Nobody said it would be impossible,” Shank says. “But there are signifi cant unknowns.”