Compiler-assisted preferred caching for embedded systems with STT-RAM based hybrid cache

As technology scales down, energy consumption is becoming a big problem for traditional SRAM-based cache hierarchies. The emerging Spin-Torque Transfer RAM (STT-RAM) is a promising replacement for large on-chip cache due to its ultra low leakage power and high storage density. However, write operations on STT-RAM suffer from considerably higher energy consumption and longer latency than SRAM. Hybrid cache consisting of both SRAM and STT-RAM has been proposed recently for both performance and energy efficiency. Most management strategies for hybrid caches employ migration-based techniques to dynamically move write-intensive data from STT-RAM to SRAM. These techniques lead to extra overheads. In this paper, we propose a compiler-assisted approach, preferred caching, to significantly reduce the migration overhead by giving migration-intensive memory blocks the preference for the SRAM part of the hybrid cache. Furthermore, a data assignment technique is proposed to improve the efficiency of preferred caching. The reduction of migration overhead can in turn improve the performance and energy efficiency of STT-RAM based hybrid cache. The experimental results show that, with the proposed techniques, on average, the number of migrations is reduced by 21.3%, the total latency is reduced by 8.0% and the total dynamic energy is reduced by 10.8%.