Refresh-aware loop scheduling for high performance low power volatile STT-RAM

The highlighted advantages of low leakage power, high storage density and immunity to electronic magnetic radiation make STT-RAM a promising candidate to build cache, SPM or main memory in embedded systems. However, write operations on STT-RAM have considerably longer latency and higher energy consumption than conventional SRAM. To solve this problem, researchers have proposed to relax STT-RAM's non-volatility and to have it work in a fast and low power mode. Under this volatile mode, refresh operations are needed to guarantee data correctness if their lifespan is larger than the retention time. It is observed that this refresh overhead is significant for data in stencil loops with the characteristic of constant read and write dependencies. This paper proposes a loop scheduling technique which can traverse loops in a new direction such that data lifespan can be greatly shortened. Therefore, overall refresh overhead can be efficiently mitigated so as to improve performance and reduce power consumption. The experimental results indicate that access latency and dynamic energy can be improved by 21.4~96.0% and 22.0~95.5% respectively by the proposed scheduling scheme.