NEMO: an energy-efficient hybrid main memory system for mobile devices.

Mobile devices run on small batteries with limited capacities. Hence, energy consumption is an important factor that determines the duration of a mobile device's usage. Main memory consumes a significant portion of the total system energy especially when the mobile device is idle, which is the case most of the times. Moreover, in current and future systems, it is very likely that users tend to run various mobile applications simultaneously, which further increases the aggregate memory demands of mobile devices. Therefore, optimizations on memory energy consumption have become even more critical in those devices. In this paper, we propose a novel scheme called NEMO to improve the energy efficiency of mobile devices with a hybrid DRAM/PCM main memory. Our scheme powers off as many power-hungry DRAM components as possible, as long as it does not impact the performance. Specifically, when the mobile device is in idle state, only a selective set of data that is critical to performance is kept in a single DRAM rank, while the rest of data is stored in PCM, so that the remaining DRAM ranks can be powered off. To do so, NEMO classifies memory pages based on their usage frequency and recency into hot and cold. Then, it places the hot pages that are more likely to be reused in future in the DRAM, which has lower access latency compared to PCM, and stores cold memory pages in PCM, which has near-zero idle power. In addition, NEMO predicts the number of DRAM ranks that need to be powered on when the mobile device becomes active for further energy saving. Our simulation results reveal that NEMO achieves on average, 10.2% reduction in memory power consumption and 1.7% performance improvement on a system running various combinations of Moby benchmark applications with 128MB DRAM and 1GB PCM, compared with the approach that simply puts DRAM into self-refresh low-power mode during idle state.