DyReCTape: A dynamically reconfigurable cache using domain wall memory tapes

Spintronic memories offer superior density, non-volatility and ultra-low standby power compared to CMOS memories, and have consequently attracted great interest in the design of on-chip caches. Domain Wall Memory (DWM) is a spintronic memory technology with unparalleled density arising from a tape-like structure. However, such a structure involves serialized access to the bits stored in each bit-cell, resulting in increased access latency, and thereby degrading performance. Prior efforts address this challenge either by limiting the number of bits per tape, in effect sacrificing the density benefits of DWM, or through cache management policies that can only partly alleviate the shift overhead. We observe that there exists significant heterogeneity in sensitivity to cache capacity and access latency across different applications, and across distinct phases of an application. We also make the key observation that DWM tapes offer a natural mechanism to tradeoff density for access latency by limiting the number of domains of each tape that are actively used to store cache data. Based on this insight, we propose DyReCTape, a dynamically reconfigurable cache that packs maximum bits per tape and leverages the intrinsic capability of DWMs to modulate the active bits per tape with minimal overhead. DyReCTape uses a history-based reconfiguration policy that tracks the number of shift operations incurred and miss rate to appropriately tailor the capacity and access latency of the DWM cache. We further propose two performance optimizations to DyReCTape: (i) a lazy migration policy to mitigate the overheads of reconfiguration, and (ii) re-use of the portion of the cache that is unused (due to reconfiguration) as a victim cache to reduce the number of offchip accesses. We evaluate DyReCTape using applications from the PARSEC and SPLASH benchmark suites. Our experiments demonstrate that DyReCTape achieves 19.8% performance improvement over an iso-area SRAM cache and 11.7% performance improvement (due to a 3.4X reduction in the number of shifts) over a state-of-the-art DWM cache.