HoPP: Hardware-Software Co-Designed Page Prefetching for Disaggregated Memory

Memory disaggregation is a promising direction to mitigate memory contention in datacenters. To make memory disaggregation practical, prior efforts expose remote memory to applications transparently via virtual memory subsystem's swapping interface. However, due to the semantic gap between OS and applications -OS cannot know the memory accessing sequences of an application but via page faults. This approach has two limitations. First, it learns little from page faults' access history, which leads to sub-optimal prefetching predictions. Second, a page fault can still occur even if there is a prefetch-hit which leads to a large kernel overhead.

To address such limitations, our key insight is to decouple the address capturing from page faults by collecting full memory access traces in the memory controller. Using this idea, we build HoPP -a hardware-software co-designed prefetching framework. HoPP adds hardware modules to the memory controller to feed sufficient hot pages to OS in real-time, which has three benefits in HoPP's software design: 1) it improves existing prefetching algorithms with simple revamps, also offers more insights to build better policies; 2) the prefetch algorithm can run as a separate data path alongside the normal remote data path via page faults, potentially hiding the swap latency from applications, and enabling fine-grained control over prefetching behaviors; 3) the prefetch-hit overhead can be eliminated by early page table entry (PTE) injection, i.e., inject PTE for the prefetched page as soon as it returns. We implemented a proof-of-concept prototype using commodity servers along with a hardwarebased memory tracking tool called HMTT to emulate a modified memory controller. Results show that compared to Fastswap and Leap, HoPP-optimized prefetching algorithm achieves over 90% accuracy and coverage, which leads to up to 59% completion time improvement for various datacenter applications.