DOVA PRO: A Dynamic Overwriting Voltage Adjustment Technique for STT-MRAM L1 Cache Considering Dielectric Breakdown Effect

As device integration density increases exponentially as predicted by Moore's law, power consumption becomes a bottleneck for system scaling where leakage power of on-chip cache occupies a large fraction of the total power budget. Spin transfer torque magnetic random access memory (STT-MRAM) is a promising candidate to replace static random access memory (SRAM) as an on-chip last level cache (LLC) due to its ultralow leakage power, high integration density, and nonvolatility. Moreover, with the prevalence of edge computing and Internet-of-Things (IoT) applications, it can be beneficial to build a total nonvolatile cache hierarchy, including the L1 cache. However, building an L1 cache with STT-MRAM still faces severe challenges particularly because reducing its relatively high write latency by increasing write voltage can accelerate oxide breakdown of the MTJ device and threaten the L1 cache lifetime significantly due to intensive accesses. In our previous work, we proposed a dynamic overwriting voltage adjustment (DOVA) technique to deal with this challenge. In this article, we improve this technique by a DOVA promotion (DOVA PRO) technique for the STT-MRAM L1 cache, considering the cache write endurance and performance simultaneously. A high write voltage is used for performance-critical cache lines, while a low write voltage is used for other cache lines to approach an optimal tradeoff between reliability and performance. Experimental results show that the proposed technique DOVA PRO can improve cache performance by 23.5%, on average, compared to the DOVA technique. In the meantime, the average degradation of cache lifetime remains almost unchanged compared with the DOVA technique on average. Furthermore, DOVA PRO can support flexible configurations to achieve various optimization targets, such as higher performance or a longer lifetime.