Mixer: Software Enabled Wear Leveling for Morphable PCM in Embedded Systems

Phase change memory (PCM) is considered as a promising alternative of DRAM-based main memory in embedded systems. A PCM cell can be dynamically programmed to be in either multiple-level cell (MLC) mode or single-level cell (SLC) mode. With this morphable feature, we can utilize the high-density of MLC and low-latency of SLC, to satisfy various memory requirements of specific applications in embedded systems. However, compared to its SLC counterpart, the lifetime of MLC is limited. In particular, a program in embedded systems usually exhibits an extremely unbalanced write pattern, which may accelerate the wear-out of MLC cells in morphable PCM. To address this issue, this paper proposes a simple and effective wear-leveling technique, named Mixer, to enhance the lifetime of morphable PCM considering the program specific features. We first build an Integer Linear Programming (ILP) formulation to produce optimal SLC/MLC partition and data allocation, to achieve a balanced write distribution in morphable PCM with low memory access cost. The basic idea is to allocate fast SLC and MLC cells for write intensive variables and other ordinary variables, respectively. We then propose a polynomial time algorithm to achieve near-optimal results. The evaluation results show that the proposed technique can effectively extend the lifetime of morphable PCM in embedded systems compared with previous work.