Content-Aware Non-Volatile Cache Replacement

Spin-Transfer Torque Magnetoresistive Random-Access Memory (STT-MRAM) is a promising memory technology, which has high density, fast read speed, low leakage power, and non-volatility, and is suitable for multi-core on-chip last-level caches. However, the high write energy and latency, as well as less-than-desirable write endurance of STT-MRAM remain challenges. This paper proposes a new encoded content-aware cache replacement policy to reduce the total switch bits for write, lower the write energy, and improve write endurance. Instead of replacing the LRU block under the conventional pseudo-LRU replacement policy, we select a replacement block near the LRU position, which has the most similar content to the missed block. The selected replacement block can reduce the switch bits without damaging the cache performance. To avoid fetching and comparing the entire block contents, we present a novel content encoding method to encode 64-byte block using just 8 bits, each bit represents 8-byte content. The encoded bit is determined by the presence of a dominant bit value in the 8 bytes. We measure the content similarity using the Hamming distance between the encoded bits of the missed block and the replaced block. Performance evaluation demonstrates that the proposed simple content encoding method is effective with an average of 20.5% reduction in total switch bits, which results in improvement on write endurance and less write energy consumption. These improvements are accomplished with low overhead and minimum impact on the cache performance.