Real-Time Scheduling for Phase Change Main Memory Systems

Multi-core processors are effective for reducing energy consumption in computer systems, since modern multicore chips allow for power management of individual cores. However, multiple cores impose higher demand on the memory subsystem, which is extremely power hungry. In addition to the small steps towards managing power in DRAMs, Phase-Change Memory (PCM) has emerged as a low-power alternative that is especially helpful for energy-aware embedded real-time systems. However, there are three drawbacks to PCM: its high latency, high energy consumption when writing, and low endurance. In real-time systems, the impact of PCM's high access latency is of special interest, as it has a negative effect on the number of deadlines that are met by the system. In this paper, we examine the memory subsystem and add a real-time scheduler for prioritizing requests at the bottleneck resource, the PCM controller. Adding support for external priorities, we use rate monotonic (RM) and earliest deadline first (EDF) prioritization at the PCM and show that it does reduce the number of deadline misses, but not sufficiently. We examine two additional schemes for prioritizing PCM requests (critical read boosting and read over write). We show that the scheduler of the PCM controller has a significant influence on the percentage of missed deadlines: critical read boosting and read over write can reduce the percentage of missed deadlines by 80% in the best case with negligible energy overhead.