Wordwidth, instructions, looping, and virtualization: the role of sharing in absolute energy minimization

When are FPGAs more energy efficient than processors? This question is complicated by technology factors and the wide range of application characteristics that can be exploited to minimize energy. Using a wire-dominated energy model to estimate the absolute energy required for programmable computations, we determine when spatially organized programmable computations (FPGAs) require less energy than temporally organized programmable computations (processors). The point of crossover will depend on the metal layers available, the locality, the SIMD wordwidth regularity, and the compactness of the instructions. When the Rent Exponent, p, is less than 0.7, the spatial design is always more energy efficient. When p=0.8, the technology offers 8-metal layers for routing, and data can be organized into 16b words and processed in tight loops of no more than 128 instructions, the temporal design uses less energy when the number of LUTs is greater than 64K. We further show that heterogeneous multicontext architectures can use even less energy than the p=0.8, 16b word temporal case.