Core tunneling: Variation-aware voltage noise mitigation in GPUs

Voltage noise and manufacturing process variation represent significant reliability challenges for modern microprocessors. Voltage noise is caused by rapid changes in processor activity that can lead to timing violations and errors. Process variation is caused by manufacturing challenges in low-nanometer technologies and can lead to significant heterogeneity in performance and reliability across the chip. To ensure correct execution under worst-case conditions, chip designers generally add operating margins that are often unnecessarily conservative for most use cases, which results in wasted energy. This paper investigates the combined effects of process variation and voltage noise on modern GPU architectures. A distributed power delivery and process variation model at functional unit granularity was developed and used to simulate supply voltage behavior in a multicore GPU system. We observed that, just like in CPUs, large changes in power demand can lead to significant voltage droops. We also note that process variation makes some cores much more vulnerable to noise than others in the same GPU. Therefore, protecting the chip against large voltage droops by using fixed and uniform voltage guardbands is costly and inefficient. This paper presents core tunneling, a variation-aware solution for dynamically reducing voltage margins. The system relies on hardware critical path monitors to detect voltage noise conditions and quickly reacts by clock-gating vulnerable cores to prevent timing violations. This allows a substantial reduction in voltage margins. Since clock gating is enabled infrequently and only on the most vulnerable cores, the performance impact of core tunneling is very low. On average, core tunneling reduces energy consumption by 15%.