Opportunities for Optimism in Contended Main-Memory Multicore Transactions

Main-memory multicore transactional systems have achieved excellent performance using single-version optimistic concurrency control (OCC), especially on uncontended workloads. Nevertheless, systems based on other concurrency control protocols, such as hybrid OCC/ locking and variations on multiversion concurrency control (MVCC), are reported to outperform the best OCC systems, especially with increasing contention. This paper shows that implementation choices unrelated to concurrency control can explain some of these performance differences. Our evaluation shows the strengths and weaknesses of OCC, MVCC, and TicToc concurrency control under varying workloads and contention levels, and the importance of several implementation choices called basis factors . Given sensible basis factor choices, OCC performance does not collapse on high-contention TPC-C. We also present two optimization techniques, deferred updates and timestamp splitting , that can dramatically improve the high-contention performance of both OCC and MVCC. These techniques are known, but we apply them in a new context and highlight their potency: when combined, they lead to performance gains of $4.74\times$ 4.74 × for MVCC and $5.01\times$ 5.01 × for OCC in a TPC-C workload.