STT-RAM cache hierarchy design and exploration with emerging magnetic devices
Spin-transfer torque random access memory (STT-RAM) is a promising new nonvolatile technology that has good scalability, zero standby power, and radiation hardness. The use of STT-RAM in last level on-chip caches has been proposed as it significantly reduced cache leakage power as technology scales down. Having a cell area only 1/9 to 1/3 that of SRAM, this will allow for a much larger cache with the same die footprint. This will significantly improve overall system performance, especially in this multicore era where locality is crucial. However, deploying STT-RAM technology in L1 caches is challenging because write operations on STT-RAM are slow and power-consuming. In this chapter, we propose a range of cache hierarchy designs implemented entirely using STT-RAM that delivers optimal power saving and performance. In particular, our designs use STT-RAM cells with various data retention times and write performances, made possible by novel magnetic tunneling junction (MTJ) designs. For L1 caches where speed is of the utmost importance, we propose a scheme that uses fast STT-RAM cells with reduced data retention time coupled with a dynamic refresh scheme. We will show that such a cache can achieve 9.2 % in performance improvement and saves up to 30 % of the total energy when compared to one that uses traditional SRAM. For lower-level caches with relatively larger cache capacities, we propose a design that has partitions of different retention characteristics and a data migration scheme that moves data between these partitions. The experiments show that on the average, our proposed multiretention-level STT-RAM cache reduces total energy by as much as 30-70 % compared to previous single retention-level STT-RAM cache, while improving IPC performance for both 2-level and 3-level cache hierarchies.
