Write-Aware Management of NVM-based Memory Extensions.

Emerging Non-Volatile Memory (NVM) technologies, such as 3D XPoint, are expected to be in production as early as 2016. Emerging NVMs are very attractive for several reasons. First, they are non-volatile and hence incur no refresh power. Second, they are dense and promising for scaling down further. Finally, they are fast and have latencies comparable to DRAM. On the other side, using emerging NVMs as direct replacement for DRAM as the main memory is challenging. Compared to DRAM, emerging NVMs can endure a very limited number of writes per cell. Furthermore, their write latency is typically much slower and more energy consuming than DRAM, e.g., Phase Change Memory (PCM) writes are multiple of times slower than that of DRAM. An important use case for emerging NVMs is using them as fast memory extensions. Memory extensions are hidden from programmers and managed by the Operating System (OS). Any access to pages held in the memory extension will cause a page fault. Later, the memory manager moves the faulting page to DRAM and maps the page. While similar in concept to the swap file, memory extensions bypass the file system. Furthermore, memory extensions are dedicated for being used as memory and hence avoid contention with the file system. In this paper, we emulate an NVM-based memory extension and study its impact on performance on a real system. We also study how to improve its performance using OS-level prefetching. We show the importance of having the system software and the NVM controller work in concert for reducing the number of writes. Our best scheme where the system software and the NVM controller work in concert could reduce the number of writes to only 5% of the original baseline (increasing its lifetime by 20x).