iPatch: Intelligent fault patching to improve energy efficiency

Dynamic voltage and frequency scaling can provide substantial energy savings but is limited by SRAM since some cells will fail at very low voltages. Due to process variation effects, a small subset of SRAM cells will be more sensitive to voltage reduction, requiring increased margins and limiting energy savings. Since large arrays like caches are most vulnerable to cell failures, recent proposals suggest disabling failing portions of the cache to enable low voltage operation. Although such approaches save power, energy reduction is limited because reducing the effective cache size increases program runtimes. In this paper, we present iPatch, a solution to regain this lost performance and enable energy savings by exploiting the redundancy inherent in superscalar processors. By relying on existing microarchitectural structures and mechanisms to 'patch' the faulty parts of caches, we enable further energy reduction with minimal overhead and complexity. Furthermore, because no critical paths or circuits are affected by our implementation, there is no impact on normal-voltage operation. For high cell failure rates, our results show significant energy savings with iPatch as well as an 18% reduction in energy-delay product compared to prior work.