Toward Efficient Programmer-Managed Two-Level Memory Hierarchies in Exascale Computers

Future exascale systems will require very aggressive memory systems simultaneously delivering huge storage capacities and multi-TB/s bandwidths. To achieve the bandwidth targets, in-package, die-stacked memory technologies will likely be necessary. However, these integrated memories do not provide enough capacity to achieve the overall per-node memory size requirements. As a result, conventional off-package memory (e.g., DIMMs) will still be needed. This creates a "two-level memory" (TLM) organization where a portion of the machine's memory space provides high bandwidth, and the remainder provides capacity at a lower level of performance. Effective use of such a heterogeneous memory organization may require the co-design of the software applications along with the advancements in memory architecture. In this paper, we explore the efficacy of programmer-driven approaches to managing a TLM system, using three Exascale proxy applications as case studies.