An Zou, Jingwen Leng, Xin He, Yazhou Zu, Vijay Janapa Reddi, Xuan Zhang
In IEEE Transactions On Computer-aided Design Of Integrated Circuits And Systems (TCAD), 2020
In today’s manycore processors, energy loss of more than 20% may result from inherent inefficiencies of conventional power delivery system (PDS) design. By stacking multiple voltage domains in series to lower the step-down conversion ratio of the off-chip voltage regulator module (VRM) and reduce energy loss along the path of the power delivery network (PDN), voltage stacking (VS) offers a novel alternative power delivery technique to fundamentally improve power delivery efficiency (PDE). However, voltage stacking suffers from aggravated supply voltage noise from current imbalance, which hinders its adoption. In this paper, we investigate practical voltage stacking implementation in manycore processors to improve power delivery efficiency (PDE) and achieve reliable performance, while maintaining compatibility with advanced power management techniques. We first present the system configuration of a voltage-stacked manycore processor. We then systematically characterize supply voltage noise in voltage stacking, identify global and residual differential currents as its dominant contributors, and calculate the possible worst supply voltage noise. We next propose a hybrid voltage regulation solution, based on a charge-recycling off-chip voltage regulator and distributed integrated voltage regulators, to mitigate supply voltage noise effectively. We also study the compatibility of voltage stacking with higher level power management techniques. Finally, the performance of a voltage-stacked GPU system is comprehensively evaluated. Simulation results show that our approach can achieve 93.5% power delivery efficiency, reducing the power loss by 13.6% compared to conventional single-layer power delivery system.