HetCache: Synergising NVMe Storage and GPU acceleration for Memory-Efficient Analytics

Accessing input data is a critical operation in data analytics: i) slow data access significantly degrades performance, and ii) storing everything in the fastest medium, i.e., memory, incurs high operational and hardware costs. Further, while GPUs offer increased analytical performance, equipping them with correspondingly fast memory requires even more expensive memory technologies than DRAM; making memory resources even more precious. Existing GPU-accelerated engines rely on CPU memory for bigger working sets, albeit at the expense of slower execution. Such a combination of both memory and compute disaggregation, however, invalidates the assumption of existing caching mechanisms: i) the processing tier is highly heterogeneous, ii) data access bandwidth depends on the access method and compute unit, iii) with NVMe arrays, persistent storage can approach in-memory bandwidth, and iv) all these relative quantities depend on the current query and data placement. Thus, existing caching approaches waste interconnect bandwidth, cache inefficiently, and overall result in suboptimal execution times. This work proposes HetCache, a storage engine for analytical workloads that optimizes the data access paths and tunes data placement by co-optimizing for the combinations of different memories, compute devices, and queries. Specifically, we present how the increasingly complex storage hierarchy impacts analytical query processing in GPU-NVMe-accelerated servers. HetCache accelerates analytics on CPU-GPU servers for larger-than-memory datasets through proportional and access-path-aware data placement. Our prototype implementation of HetCache demonstrates a 1.14x-1.78x speedup of GPU-only execution onNVMe resident data and achieves near in-system-memory performance for hybrid CPU-GPU execution, while substantially improving memory efficiency. Overall, HetCache turns the multi-memory-node nature of such heterogeneous servers from a burden into a performance booster.