Exploring the Potential for Collaborative Data Compression and Hard-Error Tolerance in PCM Memories

Limited write endurance is the main obstacle standing in the way of using phase change memory (PCM) in future computing systems. While several wear-leveling and hard-error tolerant techniques have been proposed for improving PCM lifetime, most of these approaches assume that the underlying memory uses a very simple write traffic reduction scheme (e.g., buffering, differential writes). In particular, most PCM prototypes/chips are equipped with an embedded circuit to support differential writes (DW) - on a write, only the bits that differ between the old and new data are updated. With DW, the bit-pattern of updates in a memory block is usually random, which limits the opportunity to exploit the resulting bit pattern for lifetime enhancement at an architecture level (e.g., using techniques such as wear-leveling and hard-error tolerance). This paper focuses on this inefficiency and proposes a solution based on data compression. Employing compression can improve the lifetime of the PCM memory. Using state-of-the-art compression schemes, the size of the compressed data is usually much smaller than the original data written back to memory from the last-level cache on an eviction. By storing data in a compressed format in the target memory block, first, we limit the number of bit flips to fewer memory cells, enabling more efficient intra-line wear-leveling and error recovery, and second, the unused bits in the memory block can be reused as replacements for faulty bits given the reduced size of the (compressed) data. It can also happen that for a portion of the memory blocks, the resulting compressed data is not very small. This can be due to increased data entropy introduced by compression, where the total number of bit flips will be increased over the baseline system. In this paper, we present an approach that provides collaborative operation of data compression, differential writes, wear-leveling and hard-error tolerant techniques targeting PCM memories. We propose approaches that reap the maximum benefits from compression, while also enjoying the benefits of techniques that reduce the number of high-entropy writes. Using an approach that combines different solutions, our mechanism tolerates 2.9× more cell failures per memory line and achieves a 4.3× increase in PCM memory lifetime, relative to our baseline state-of-the-art PCM DIMM memory.