Designing the hardware

Improving communication between distributed processors and managing shared data are two of the central challenges in creating tomorrow’s chips

 With the multicore chips in today’s personal computers, which might have four or six or even eight cores, splitting computational tasks hasn’t proved a huge problem. If the chip is running four programs — say, a word processor, an e-mail program, a Web browser and a media player — the operating system can assign each its own core. But in future chips, with hundreds or even thousands of cores, a single program will be split among multiple cores, which drastically complicates things. The cores will have to exchange data much more often; but in today’s chips, the connections between cores are much slower than the connections within cores. Cores executing a single program may also have to modify the same chunk of data, but the performance of the program could be radically different depending on which of them gets to it first. At MIT, a host of researchers are exploring how to reinvent chip architecture from the ground up, to ensure that adding more cores makes chips perform better, not worse.
In August 2010, the U.S. Department of Defense’s Defense Advanced Research Projects Agency announced that it was dividing almost $80 million among four research teams as part of a “ubiquitous high-performance computing” initiative. Three of those teams are led by commercial chip manufacturers. The fourth, which includes researchers from Mercury Computer, Freescale, the University of Maryland and Lockheed Martin, is led by MIT’s Computer Science and Artificial Intelligence Lab and will concentrate on the development of multicore systems.
One way to improve communication between cores, which the Angstrom project is investigating, is optical communication — using light instead of electricity to move data. Though prototype chips with optical-communications systems have been built in the lab, they rely on exotic materials that are difficult to integrate into existing chip-manufacturing processes. Two of the Angstrom researchers are investigating optical-communications schemes that use more practical materials.
In early 2010, an MIT research group led by Lionel Kimerling, the Thomas Lord Professor of Materials Science and Engineering, demonstrated the first germanium laser. Germanium is already used in many commercial chips simply to improve the speed of electrical circuits, but it has much better optical properties than silicon. Another Angstrom member, Vladimir Stojanović of the Microsystems Technology Laboratory, is collaborating with several chip manufacturers to build prototype chips with polysilicon waveguides. Waveguides are ridges on the surface of a chip that can direct optical signals; polysilicon is a type of silicon that consists of tiny, distinct crystals of silicon clumped together. Typically used in the transistor element called the gate, polysilicon has been part of the standard chip-manufacturing process for decades.
Other Angstrom researchers, however, are working on improving electrical connections between cores. In today’s multicore chips, adjacent cores typically have two high-capacity connections between them, which carry data in opposite directions, like the lanes of a two-lane highway. But in future chips, cores’ bandwidth requirements could fluctuate wildly. A core performing a calculation that requires information updates from dozens of other cores would need much more receiving capacity than sending. But once it completes its calculation, it might have to broadcast the results, so its requirements would invert. Srini Devadas, a professor in the Computer Science and Artificial Intelligence Lab, is researching chip designs in which cores are connected by eight or maybe 16 lower-capacity connections, each of which can carry data in either direction. As the bandwidth requirements of the chip change, so can the number of connections carrying data in each direction. Devadas has demonstrated that small circuits connected to the cores can calculate the allotment of bandwidth and switch the direction of the connections in a single clock cycle.
In theory, a computer chip has two main components: a processor and a memory circuit. The processor retrieves data from the memory, performs an operation on it, then returns it to memory. But in practice, chips have for decades featured an additional, smaller memory circuit called a cache, which is closer to the processor, can be accessed much more rapidly than main memory, and stores frequently used data. The processor might perform dozens or hundreds of operations on a chunk of data in the cache before relinquishing it to memory.