2016 USENIX Annual Technical Conference (USENIX ATC '16)

We propose a new HTM-assisted concurrency control protocol, called HTCC, that achieves high scalability and robustness when processing OLTP workloads. HTCC attains its goal using a two-pronged strategy that exploits the strengths of HTM. First, it distinguishes between hot and cold records, and deals with each type differently – while accesses to highly contended data are protected using conventional fine-grained locks, accesses to cold data are HTM-guarded. This remarkably reduces the database transaction abort rate and exploits HTM’s effectiveness in executing low-contention critical sections. Second, to minimize the overhead inherited from successive restarts of aborted database transactions, HTCC caches the internal execution states of a transaction for performing delta-restoration, which partially updates the maintained read/write set and bypasses redundant index lookups during transaction re-execution at best effort. This approach is greatly facilitated by HTM’s speedy hardware mechanism for ensuring atomicity and isolation. We evaluated HTCC in a main-memory database prototype running on a 4 socket machine (40 cores in total), and confirmed that HTCC can scale near-linearly, yielding high transaction rate even under highly contended workloads.