Analyzing the impact of useless write-backs on the endurance and energy consumption of PCM main memory

Phase Change Memory (PCM) is an emerging technology that has been recently considered as a cost-effective and energy-efficient alternative to traditional DRAM main memory. Due to the high energy consumption of writes and limited number of write cycles, reducing the number of writes to PCM can result in considerable energy savings and endurance improvement. In this paper, we introduce the concept of useless write-backs, which occur when a dirty cache line that belongs to a dead memory region is evicted from the cache (a dead region is a memory location that is not used again by a program). Since the evicted data is not used again, the write-back can be safely avoided to improve endurance and energy consumption. This paper presents a limit study on the improvement that passing information to the memory system about useless writebacks has on the endurance and energy consumption of systems based on PCM main memory. We developed algorithms to measure the number of useless write-backs to PCM for three different types of memory regions and we present an energy model to determine the maximum energy savings that could potentially be achieved through such a scheme. Our results show that avoiding useless write-backs can save up to 19.8% of energy and improve endurance by up to 26.2%.