NPC: A Non-Conflicting Processing-in-Memory Controller in DDR Memory Systems

Seungyong Lee, Sanghyun Lee, Minseok Seo, Chunmyung Park, Woojae Shin, Hyuk-Jae Lee, Hyun Kim

Abstract: Processing-in-Memory (PIM) has emerged as a promising solution to address the memory wall problem. Existing memory interfaces must support new PIM commands to utilize PIM, making the definition of PIM commands according to memory modes a major issue in the development of practical PIM products. For performance and OS-transparency, the memory controller is responsible for changing the memory mode, which requires modifying the controller and resolving conflicts with existing functionalities. Additionally, it must operate to minimize mode transition overhead, which can cause significant performance degradation. In this study, we present NPC, a memory controller designed for mode transition PIM that delivers PIM commands via the DDR interface. NPC issues PIM commands while transparently changing the memory mode with a dedicated scheduling policy that reduces the number of mode transitions with aggregative issuing. Moreover, existing functions, such as refresh, are optimized for PIM operation. We implement NPC in hardware and develop a PIM emulation system to validate it on FPGA platforms. Experimental results reveal that NPC is compatible with existing interfaces and functionality, and the proposed scheduling policy improves performance by 2.2$\boldsymbol{\times}$ with balanced fairness, achieving up to 97% of the ideal performance. These findings have the potential to aid the application of PIM in real systems and contribute to the commercialization of mode transition PIM.