MigrantStore: Leveraging Virtual Memory in DRAM-PCM Memory Architecture

With the imminent slowing down of DRAM scaling, Phase Change Memory (PCM) is emerging as a lead alternative for main memory technology. While PCM achieves low energy due to various technology-specific advantages, PCM is significantly slower than DRAM (especially for writes) and can endure far fewer writes before wearing out. Previous work has proposed to use a large, DRAM-based hardware cache to absorb writes and provide faster access. However, due to ineffectual caching where blocks are evicted before sufficient number of accesses, hardware caches incur significant overheads in energy and bandwidth, two key but scarce resources in modern multicores. Because using hardware for detecting and removing such ineffectual caching would incur additional hardware cost and complexity, we leverage the OS virtual memory support for this purpose. We propose a DRAM-PCM hybrid memory architecture where the OS migrates pages on demand from the PCM to DRAM. We call the DRAM part of our memory as MigrantStore which includes two ideas. First, to reduce the energy, bandwidth, and wear overhead of ineffectual migrations, we propose migration hysteresis. Second, to reduce the software overhead of good replacement policies, we propose recentlyaccessed-page-id (RAPid) buffer, a hardware buffer to track the addresses of recently-accessed MigrantStore pages.