A supercomputer is composed of processors, memory, I/O system, and an interconnect. The processors fetch and execute program instructions. This execution involves performing arithmetic and logical calculations, initiating memory accesses, and controlling the flow of program execution. The memory system stores the current state of a computation. A processor or a group of processors (an SMP) and a block of memory are typically packaged together as a node of a computer. A modern supercomputer has hundreds to tens of thousands of nodes. The interconnect provides communication among the nodes of the computer, enabling these nodes to collaborate on the solution of a single large problem. The interconnect also connects the nodes to I/O devices, including disk storage and network interfaces. The I/O system supports the peripheral subsystem, which includes tape, disk, and networking. All of these subsystems are needed to provide the overall system. Another aspect of providing an overall system is power consumption. Contemporary supercomputer systems, especially those in the top 10 of the TOP500, consume in excess of 5 megawatts. This necessitates the construction of a new generation of supercomputer facilities (e.g., for the Japanese Earth Simulator, the Los Alamos National Laboratory, and the Lawrence Livermore National Laboratory). Next-generation petaflops systems must consider power consumption in the overall design.

As semiconductor and packaging technology improves, different aspects of a supercomputer (or of any computer system) improve at different rates. In particular, the arithmetic performance increases much faster than the local and global bandwidth of the system. Latency to local memory or to a remote node is decreasing only very slowly. When expressed in terms of instructions executed in the time it takes to communicate to local memory or to a remote node, this latency is increasing rapidly. This nonuniform scaling of technology poses a number of challenges for supercomputer architecture, particularly for those applications that demand high local or global bandwidth.

Figure 5.1 shows how floating-point performance of commodity microprocessors, as measured by the SPECfp benchmark suite, has scaled over time.¹ The trend line shows that the floating-point performance of