Design techniques to improve the device write margin for MRAM-based cache memory

As one promising non-volatile memory technology, magnetoresistive RAM (MRAM) based on magnetic tunneling junctions (MTJs) has recently attracted much attention. However, latest device research has discovered that, in order to maintain sufficient MTJ write margin to prevent device breakdown, MTJs will be subject to unconventionally high random write error rates (e.g., 10^-3 and above) as memory cell size is being scaled down. This new discovery seriously threatens the scalability of MRAM, and the material/device research community is actively searching for solutions to largely reduce MTJ write error rates and meanwhile maintain sufficient device write margin. In this paper, we attempt to address this challenge from the architecture level when using MRAM to implement cache memory. In particular, we show that two simple cache architecture design techniques can be used to effectively tolerate high MTJ write error rates at small performance and implementation cost, which makes it much easier to maintain sufficient MTJ write margin and hence push the MRAM scalability envelope. Using the full system simulator PTLsim and a variety of benchmarks, we show that the proposed design techniques can readily accommodate MTJ write error rate up to 0.75% at the penalty of less than 4% processor performance degradation, less than 10% silicon area overhead, and 6% energy consumption overhead.