Variation Aware Energy-Efficient Methodologies for Homogeneous Many-core Designs

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Virginia Tech


Earlier designs were driven by the goal of achieving higher performance, but lately, energy efficiency has emerged as an even more important design principle. Strong demand from the consumer electronics drives research in the low power and energy-efficient methodologies. Moreover, with exponential increase in the number of transistors on a chip and with further technology scaling, variability in the design is now of greater concern. Variations can make the design unreliable or the design may suffer from sub-optimal performance. Through the work in this thesis, we present a multi-dimensional investigation into the design of variation aware energy-efficient systems. Our overarching methodology is to use system-level decisions to mitigate undesired effects originating from device-level and circuit-level issues.

We first look into the impact of process variation (PV) on energy efficient, scalable throughput many-core DSP systems. In our proposed methodology, we leverage the benefits of aggressive voltage scaling (VS) for obtaining energy efficiency while compensating for the loss in performance by exploiting parallelism present in various DSP designs. We demonstrate this proposed methodology consumes 8% - 77% less power as compared to simple dynamic VS over different workload environments. Later, we show judicious system-level decisions, namely, number of cores, and their operating voltage can greatly mitigate the effects of PV and consequently, improve the energy efficiency of the design. We also present our analysis discussing the impact of aging on the proposed methodology. To validate our proposed system-level approach, design details of a prototype chip fabricated in the 90nm technology node and its findings are also presented. The chip consists of 8 homogeneous FIR cores, which are capable of running from near-threshold to nominal voltages. In the 20-chip population, we observe 7% variation in the speed at nominal voltage (0.9V) and 26% at near threshold voltage (0.55V) among all the cores. We also observe 54% variation in power consumption characteristics of the cores. The chip measurement results show that our proposed methodology of judiciously selecting the cores and their operating voltage can result in 6.27% - 28.15% more energy savings for various workload environments, as compared to globally voltage scaled systems. Furthermore, we present the impact of temperature variations on the energy-efficiency of the above systems.

We also study the problem of voltage variations in the integrated circuits. We first present the characteristics of a dynamic voltage noise as measured on a 28nm FPGA. We propose a fully digital on-chip sensor that can detect the fast voltage transients and alert the system of voltage emergency. A traditional approach to mitigate this problem is to use safety guardbands. We demonstrate that our proposed sensor system will be 6% - 27.5% more power efficient than the traditional approach.



Low power, Near-threshold, Energy efficient, di/dt, process, variation, PVTA, Aging, homogeneous many-core