The Roadrunner supercomputer provides a best-in-the-world tool for enhancing our scientific understanding of nuclear weapons and ensuring the reliability and safety of the U.S. nuclear stockpile.

In May 2008, the Roadrunner supercomputer—a partnership between Los Alamos National Laboratory in Los Alamos, New Mexico, the National Nuclear Security Administration and IBM—became the first computer to perform one quadrillion calculations per second. That’s a petaflop—1,000 times faster than the fastest supercomputer a decade ago.

Roadrunner provides the first full-scale example of the future of high-performance computing. Its innovative design uses the low-power, high-performance PowerXCell 8i processor to achieve unprecedented performance at a relatively modest energy cost. Roadrunner is more than twice as energy efficient as the Jaguar supercomputer at Oak Ridge National Laboratory in Oak Ridge, Tennessee (see “The Supercomputing Fast Lane,” page 112), a more conventional design. Roadrunner was cited as one of 2008’s top 10 inventions by Time magazine.

Roadrunner’s fleet-footed numeric modeling capabilities will focus on classified research to maintain the nation’s nuclear weapons stockpile. The supercomputer’s speed will be used to increase our predictive capability and improve confidence in predicting the behavior of nuclear weapons. These efforts will include predicting and understanding the behavior of thermonuclear burning plasmas and laser-plasma interactions in experiments at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, in Livermore, California. Charles McMillan, associate director for weapons physics at Livermore, expects that “Roadrunner will provide a best-in-theworld computational tool for enhancing our understanding of nuclear weapons science and generating a science-based predictive capability for ensuring the reliability and safety of the nuclear stockpile.”

Most nuclear weapons in the U.S. stockpile were produced 30 to 40 years ago, and no new nuclear weapons have been produced since the end of the Cold War. Since underground nuclear testing ended in 1992, the United States has relied on sciencebased research and development to extend the lifetime of the weapons in the existing stockpile.

The Roadrunner supercomputer uses commercially available hardware and software, including components applicable to commercial video game platforms. The system’s computational power comes from the PowerXCell 8i, an IBM-developed derivative of the Cell processor in Sony’s PlayStation 3 (PS3). The PS3 Cell processor was produced as part of a collaboration between Sony, Toshiba and IBM. Early in the process scientists from Livermore and IBM recognized that the Cell design needed additional capabilities for numerically intensive computing. By adding these capabilities, the Roadrunner project leveraged the original $400 million investment.

The secret to Roadrunner’s record-breaking performance is a unique hybrid design, the first of its kind. Each of more than 3,000 nodes in the system contains two different processors: two AMD Opteron dual-core processors and four PowerXCell processors. The PowerXCell also is a hybrid, having one standard IBM Power PC microprocessor core and eight vector processor cores, which provide this chip’s remarkable performance and efficiency.

The full Roadrunner system occupies about 5,000 square feet and uses about 2.4 megawatts of energy while running at top speed. For future supercomputers, power consumption is potentially a limiting factor. However, Roadrunner and other systems such as the IBM’s Blue Gene/P system at Argonne National Laboratory in DuPage County, Illinois, are breaking down this barrier. Roadrunner uses about the same amount of power as Livermore’s most recently retired supercomputer while providing 50 times the performance.

Future generations of supercomputers will be programmed differently from those currently in operation. New algorithms and tools will be required to gain access to their incredible performance. Roadrunner is the first of this new generation. In concert with the Roadrunner project, IBM has developed the Software Development Kit for Multicore Acceleration, which provides a new set of tools for programming the system. Los Alamos scientists demonstrated that Roadrunner could be effectively programmed and used by focusing on several important applications: plasma physics, materials science and particle transport.