Using a Flexible Fault-Tolerant Cache (FFT-Cache) to Improve Reliability in Ultra Low Voltage Operation

Caches are known to consume a large part of total microprocessor power. Traditionally, voltage scaling has been used to reduce both dynamic and leakage power in caches. However, aggressive voltage reduction causes process-variation-induced failures in cache SRAM arrays, which compromise cache reliability. In this paper, we propose Flexible Fault-Tolerant Cache (FFT-Cache) that uses a flexible defect map to configure its architecture to achieve significant reduction in energy consumption through aggressive voltage scaling, while maintaining high error reliability. FFT-Cache uses a portion of faulty cache blocks as redundancy – using block-level or line-level replication within or between sets –to tolerate other faulty caches lines and blocks. Our configuration algorithm categorizes the cache lines based on degree of conflict of their blocks to reduce the granularity of redundancy replacement. FFT-Cache thereby sacrifices a minimal number of cache lines to avoid impacting performance while tolerating the maximum amount of defects. Our experimental results on SPEC2K benchmarks demonstrate that the operational voltage is reduced down to 375mV, which achieves up to 80% reduction in dynamic power and up to 48% reduction in leakage power with small performance impact and area overhead.