DWM-TAPESTRI - An energy efficient all-spin cache using domain wall shift based writes

Spin-based memories are promising candidates for future on-chip memories due to their high density, non-volatility, and very low leakage. However, the high energy and latency of write operations in these memories is a major challenge. In this work, we explore a new approach - shift based write - that offers a fast and energy-efficient alternative to performing writes in spin-based memories. We propose DWM-TAPESTRI, a new all-spin cache design that utilizes Domain Wall Memory (DWM) with shift based writes at all levels of the cache hierarchy. The proposed write scheme enables DWM to be used, for the first time, in L1 caches and in tag arrays, where the inefficiency of writes in spin memories has traditionally precluded their use. At the circuit level, we propose bit-cell designs utilizing shift-based writes, which are tailored to the differing requirements of different levels in the cache hierarchy. We also propose pre-shifting as an architectural technique to hide the latency of shift operations that is inherent to DWM. We performed a systematic device-circuit-architecture evaluation of the proposed design. Over a wide range of SPEC 2006 benchmarks, DWM-TAPESTRI achieves 8.2X improvement in energy and 4X improvement in area, with virtually identical performance, compared to an iso-capacity SRAM cache. Compared to an iso-capacity STT-MRAM cache, the proposed design achieves around 1.6X improvement in both area and energy under iso-performance conditions.