A Novel Hybrid Last Level Cache Based on Multi-retention STT-RAM Cells

Spin-transfer torque random access memory (STT-RAM) is one of the most promising substitutes for universal main memory and cache due to its excellent scalability, high storage density and low leakage power. A much larger cache capacity in the same die footprint can be implemented with STT-RAM because its area is only 1/9 to 1/3 that of SRAM. However, the non-volatile STT-RAM also has some drawbacks, such as long write latency and high write energy, which limit its application in cache design. To solve the two problems, we relax the retention time of STT-RAM to optimize its write performance and energy, and propose a novel multi-retention STT-RAM hybrid last level cache (LLC) architecture, which is realized with three different kinds of cells. In addition, we design the data migration scheme to manage its block allocation, thus improving overall system performance further. The experiment results show that our multi-retention hybrid LLC reduces the total power consumption by as much as 96.6 % compared with SRAM LLC, while having almost the same (at 99.4 %) instruction per cycle (IPC).