ADT: Aggressive Demotion and Promotion for Tiered Memory

Tiered memory using DRAM as upper-tier (fast memory) and emerging slower-but-larger byte-addressable memory as lower-tier (slow memory) is a promising approach to expanding main-memory capacity. Based on the observation that there are many cold pages in data-center applications,
proactive demotion
schemes demote cold pages to slow memory even when free space in fast memory is not deficient. Prior works on proactive demotion lower the requirement of expensive fast-memory capacity by reducing applications’ resident set size in fast memory. Also, some of the prior works mitigate the massive performance drop due to insufficient fast-memory capacity when there is a spike in demand for hot data. However, there is room for further improvement to save a larger fast-memory capacity with further aggressive demotion, which can fully reap the aforementioned advantages of proactive demotion. In this paper, we propose a new proactive demotion scheme, ADT, which performs
a
ggressive
d
emotion and promotion for
t
iered memory. Using the memory access locality within the unit in which applications and memory allocators allocate memory, ADT extends the unit of demotion/promotion from the page adopted by prior works to make its demotion more aggressive. By performing demotion and promotion by the extended unit, ADT reduces 29% of fast-memory usage with only a 2.3% performance drop. Also, it achieves 2.28× speedup compared to the default Linux kernel when the system's memory usage is larger than fast-memory capacity, which outperforms state-of-the-art schemes for tiered memory management.