Scalable garbage collection for in-memory MVCC systems

Abstract

To support Hybrid Transaction and Analytical Processing (HTAP), database systems generally rely on Multi-Version Concurrency Control (MVCC). While MVCC elegantly enables lightweight isolation of readers and writers, it also generates outdated tuple versions, which, eventually, have to be reclaimed. Surprisingly, we have found that in HTAP workloads, this reclamation of old versions, i.e., garbage collection, often becomes the performance bottleneck. It turns out that in the presence of long-running queries, state-of-the-art garbage collectors are too coarse-grained. As a consequence, the number of versions grows quickly slowing down the entire system. Moreover, the standard background cleaning approach makes the system vulnerable to sudden spikes in workloads. In this work, we propose a novel garbage collection (GC) approach that prunes obsolete versions eagerly. Its seamless integration into the transaction processing keeps the GC overhead minimal and ensures good scalability. We show that our approach handles mixed workloads well and also speeds up pure OLTP workloads like TPC-C compared to existing state-of-the-art approaches.