Characterizing memory write references for efficient management of hybrid PCM and DRAM memory

In order to reduce the energy dissipation in main memory of computer systems, phase change memory (PCM) has emerged as one of the most promising technologies to incorporate into the memory hierarchy. However, PCM has two critical weaknesses to substitute DRAM memory in its entirety. First, the number of write operations allowed to each PCM cell is limited. Second, write access time of PCM is about 6-10 times slower than that of DRAM. To cope with this situation, hybrid memory architectures that use a small amount of DRAM together with PCM memory have been suggested. In this paper, we present a new memory management technique for hybrid PCM and DRAM memory architecture that efficiently hides the slow write performance of PCM. Specifically, we aim to estimate future write references accurately and then absorb most memory writes into DRAM. To do this, we analyze the characteristics of memory write references and find two noticeable phenomena. First, using write history alone performs better than using both read and write history in estimating future write references. Second, the frequency characteristic is a better estimator than temporal locality but combining these two properties appropriately leads to even better results. Based on these two observations, we present a new page replacement algorithm called CLOCK-DWF (CLOCK with Dirty bits and Write Frequency) that significantly reduces the number of write operations that occur on PCM.