We propose overcoming the memory capacity limitation of GPUs with high-capacity Storage-Class Memory (SCM) and DRAM cache. By significantly increasing the memory capacity with SCM, the GPU can capture a larger fraction of the memory footprint than HBM for workloads that mandate memory oversubscription, resulting in substantial speedups. However, the DRAM cache needs to be carefully designed to address the latency and bandwidth limitations of the SCM while minimizing cost overhead and considering GPU's characteristics. Because the massive number of GPU threads can easily thrash the DRAM cache and degrade performance, we first propose an SCM-aware DRAM cache bypass policy for GPUs that considers the multi-dimensional characteristics of memory accesses by G PU s with SCM to bypass DRAM for data with low performance utility. In addition, to reduce DRAM cache probe traffic and increase effective DRAM BW with minimal cost overhead, we propose a Configurable Tag Cache (CTC) that repurposes part of the L2 cache to cache DRAM cacheline tags. The L2 capacity used for the CTC can be adjusted by users for adaptability. Furthermore, to minimize DRAM cache probe traffic from CTC misses, our Aggregated Metadata-In-Last-column (AMIL) DRAM cache organization co-locates all DRAM cacheline tags in a single column within a row. The AMIL also retains the full ECC protection, unlike prior DRAM cache implementation with Tag-And-Data (TAD) organization. Additionally, we propose SCM throttling to curtail power consumption and exploiting SCM's SLC/MLC modes to adapt to workload's memory footprint. While our techniques can be used for different DRAM and SCM devices, we focus on a Heterogeneous Memory Stack (HMS) organization that stacks SCM dies on top of DRAM dies for high performance. Compared to HBM, the HMS improves performance by up to 12.5× (2.9× overall) and reduces energy by up to 89.3% (48.1 % overall). Compared to prior works, we reduce DRAM cache probe and SCM write traffic by 91–93 % and 57–75 %, respectively.

Emerging Compute Express Link (CXL) enables cost-efficient memory expansion beyond the local DRAM of processors. While its CXL.mem protocol provides minimal latency overhead through an optimized protocol stack, frequent CXL memory accesses can result in significant slowdowns for memory-bound applications whether they are latency-sensitive or bandwidth-intensive. The near-data processing (NDP) in the CXL controller promises to overcome such limitations of passive CXL memory. However, prior work on NDP in CXL memory proposes application-specific units that are not suitable for practical CXL memory-based systems that should support various applications. On the other hand, existing CPU or GPU cores are not cost-effective for NDP because they are not optimized for memory-bound applications. In addition, the communication between the host processor and CXL controller for NDP offloading should achieve low latency, but existing CXL.io/PCIe-based mechanisms incur$\mu\mathbf{s}-\mathbf{scale}$latency and are not suitable for fine-grained NDP.