Characterizing the Overhead of Software-Managed Hybrid Main Memory

The size of main memory in modern computers is approaching energy and scalability limits. Combining DRAM and non-volatile memory (NVM) has been proposed to increase capacity and reliability, and to decrease energy consumption. Software-managed hybrid memory is a promising way to incorporate NVM in main memory due to its architectural simplicity. However, there are significant performance issues caused by interference due to data migration between DRAM and NVM and a lack of effective migration policies. To aid in the development of migration policies and hardware mechanisms for incorporating NVM in main memory, we propose new analysis and simulation techniques to understand the behavior of software-managed hybrid memory. These techniques allow us to characterize the overhead experienced by requests in the memory hierarchy and identify the factors that limit performance in software-managed hybrid memory. Using our techniques, we show that queuing delays at the NVM banks and NVM bus are the main limiting factors, and that there is significant potential to improve performance with better migration policies.