But the hunger for ever-increasing network bandwidth is never satiated, leading to the introduction of Infiniband, which can operate at speeds of up to 96Gbps – nearly a hundred times faster than the Gigabit Ethernet used for more general networking. The IBM Roadrunner uses Infiniband to connect its clusters.
A 100Gbps version of Ethernet called 100Gbase-X is also under development. Some supercomputer manufacturers have developed their own proprietary interconnect technology. NEC's IXS Super-Switch technology offers a staggering 256Gbps.
Another perennial problem of performance computing is that processing power also requires electrical power. This means that the more of the former you want, the more of the latter you're going to need.
IBM's valve-based AN/FSQ-7 of the 1950s required as much as 3 MegaWatts – enough to illuminate a small town. The headline figures haven't diminished much over the years, either, with IBM's Roadrunner requiring 2.35MW at peak – although Roadrunner packs in thousands of processors while its predecessor powered just one.
Closely associated with this hunger for Watts is one of its by-products: heat. Cray tackled this situation from the outset, using liquid cooling achieved with Freon and copper cold plates. The company also developed some other novel cooling systems, such as immersing components in electrically inert but highly heat-conductive fluids. This method was used to cool the Cray-2.
But the problem of cooling supercomputers extends far beyond its main internal components. With MegaWatts of electrical power going in, getting the heat away from the circuitry is just the beginning.
The cabinets must be designed with heat dissipation in mind, and the whole architecture of the supercomputer facility must transfer hot air to the outside atmosphere. This generally involves hefty amounts of air conditioning.
Some designs have involved elaborate water-cooling pipes worked through the facility itself, although this has fallen out of favour for cost reasons. Either way, taming the thermal problem is likely to consume a significant amount of electrical power. For example, IBM's ASCI White required 3MW to power its computing tasks, but it required an equal amount of power to cool the system while it was running.
Meeting the challenges
Most of the fastest computers in the world now use similar processors to those found in your desktop PC and even the latest consoles.
Cray's XT Jaguar came close to beating IBM's Roadrunner with a massive array of 45,000 quad-core AMD Opterons. But there is research into new designs that could again increase the power of individual processors by orders of magnitude. For example, CPUs are still resolutely two-dimensional.
Since getting data around the various components is a major issue for massively parallel systems, being able to pack transistors on top of each other as well as side by side promises the kind of leap in performance caused by the integrated circuit itself.
In October 2008, the Interuniversity Microelectronics Centre (IMEC) in Belgium announced a breakthrough in 3D stacking, demonstrating working circuits using its 5μm copper through-silicon vias (Cu-TSV) process. Two 130nm wafers were sandwiched on top of each other, with copper lands bonded together using thermocompression. So, in theory, two quad-core processors could be packaged into the space of a single eight-core processor.
In Japan, electronics firm Unisantis is working on a Stacked-Surrounding Gate Transistor (S-SGT) design, which promises to enable chips with clockspeeds between 20GHz and 50GHz. S-SGT is a bit like perpendicular recording in hard disks, with the transistors arranged vertically rather than horizontally.
This means that more transistors can be packed into the same space, and it reduces both the effects of some of the unwanted physical properties that are encountered when transistors reach a certain level of miniaturisation (such as gate leakage) and the speed limits caused by how far electrons have to travel from gate to gate.
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