Enabling technologies for memory compression: Metadata, mapping, and prediction

Future systems dealing with big-data workloads will be severely constrained by the high performance and energy penalty imposed by data movement. This penalty can be reduced by storing datasets in DRAM or NVM main memory in compressed formats. Prior compressed memory systems have required significant changes to the operating system, thus limiting commercial viability. The first contribution of this paper is to integrate compression metadata with ECC metadata so that the compressed memory system can be implemented entirely in hardware with no OS involvement. We show that in such a system, read operations are unable to exploit the benefits of compression because the compressibility of the block is not known beforehand. To address this problem, we introduce a compressibility predictor that yields an accuracy of 97%. We also introduce a new data mapping policy that is able to maximize read/write parallelism and NVM endurance, when dealing with compressed blocks. Combined, our proposals are able to eliminate OS involvement and improve performance by 7% (DRAM) and 8% (NVM), and system energy by 12% (DRAM) and 14% (NVM), relative to an uncompressed memory system.