Current high-performance computer systems utilize a memory hierarchy of on-chip cache, main memory, and secondary storage due to differences in device characteristics. Limiting the amount of main memory causes page swap operations and duplicates data between the main memory and the storage device. The characteristics of next-generation memory, such as nonvolatility, byte addressability, and scaling to greater capacity, can be used to solve these problems. Simple replacement of secondary storage with new forms of nonvolatile memory in a traditional memory hierarchy still causes typical problems, such as memory bottleneck, page swaps, and write overhead. Thus, we suggest a single architecture that merges the main memory and secondary storage into a system called a Memory-Disk Integrated System (MDIS). The MDIS architecture is composed of a virtually decoupled NVRAM and a nonvolatile memory performance optimizer combining hardware and software to support this system. The virtually decoupled NVRAM module can support conventional main memory and disk storage operations logically without data duplication and can reduce write operations to the NVRAM. To increase the lifetime and optimize the performance of this NVRAM, another hardware module called a Nonvolatile Performance Optimizer (NVPO) is used that is composed of four small buffers. The NVPO exploits spatial and temporal characteristics of static/dynamic data based on program execution characteristics. Enhanced virtual memory management and address translation modules in the operating system can support these hardware components to achieve a seamless memory-storage environment. Our experimental results show that the proposed architecture can improve execution time by about 89% over a conventional DRAM main memory/HDD storage system, and 77% over a state-of-the-art PRAM main memory/HDD disk system with DRAM buffer. Also, the lifetime of the virtually decoupled NVRAM is estimated to be 40% longer than that of a traditional hierarchy based on the same device technology.

This paper presents a new memory hierarchy system using next-generation non-volatile memory devices merging a conventional main memory layer and a disk storage layer into a single memory layer, called a memory-disk integrated system (MDIS). In the MDIS, files are stored in MDIS storage and accessed in place as if they are in conventional DRAM main memory without any duplication between the main memory and secondary storage. The MDIS comprises three components: a virtual accessing adapter (VAA), an array of non-volatile hybrid memory (NVHM), and its associated virtual memory management module. The NVHM can be divided into two spaces-specifically, static and dynamic spaces-to support conventional memory management. Further, the VAA comprises four small buffers that support static and dynamic virtualization according to program execution characteristics. Experimental results show that the read/write load traffics for the proposed MDIS can be reduced by about 73% as compared with the cases of conventional main memory system. In addition, access latency of non-volatile hybrid memory is reduced by 51%.