SELCC: Coherent Caching over Compute-Limited Disaggregated Memory

Disaggregating memory from compute offers the opportunity to better utilize stranded memory in data centers. It is important to cache data in the compute nodes and maintain cache coherence across multiple compute nodes to save on round-trip communication cost between the disaggregated memory and the compute nodes. However, the limited computing power on the disaggregated memory servers makes it challenging to maintain cache coherence among multiple compute-side caches over disaggregated shared memory. This paper introduces SELCC; a Shared-Exclusive Latch Cache Coherence protocol that maintains cache coherence without imposing any computational burden on the remote memory side. SELCC builds on a one-sided shared-exclusive latch protocol by introducing lazy latch release and invalidation messages among the compute nodes so that it can guarantee both data access atomicity and cache coherence. SELCC minimizes communication round-trips by embedding the current cache copy holder IDs into RDMA latch words and prioritizes local concurrency control over global concurrency control. We instantiate the SELCC protocol onto compute-sided cache, forming an abstraction layer over disaggregated memory. This abstraction layer provides main-memory-like APIs to upper-level applications, and thus enabling existing data structures and algorithms to function over disaggregated memory with minimal code change. To demonstrate the usability of SELCC, we implement a B-tree and three transaction concurrency control algorithms over SELCC's APIs. Micro-benchmark results show that the SELCC protocol achieves better performance compared to RPC-based cache-coherence protocols. Additionally, YCSB and TPC-C benchmarks indicate that applications over SELCC can achieve comparable or superior performance against competitors over disaggregated memory.