ACM Computing Surveys

Concurrency control: Methods, performance, and analysis

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Standard locking (two-phase locking with on-demand lock requests and blocking upon lock conflict) is the primary concurrency control (CC) method for centralized databases. The main source of performance degradation with standard locking is blocking, whereas transaction (txn) restarts to resolve deadlocks have a secondary effect on performance. We provide a performance analysis of standard locking that accurately estimates its performance degradation leading to thrashing. We next introduce two sets of methods to cope with its performance limitations. Restart-oriented locking methods selectively abort txns to increase the level of concurrency for active txns with respect to standard locking in high-contention environments. For example, the running-priority method aborts blocked txns based on the essential blocking principle, which only allows blocking by active txns. The wait-depth-limited (WDL) method further minimizes wasted processing by basing abort decisions on the progress made by a txn. Restart waiting serves as a load-control mechanism by deferring the restart of an aborted txn until conflicting txns have left the system. These two methods have performance superior to other restart-oriented methods and standard locking in high-contention environments. In two-phase processing methods an aborted txn may continue its first phase of execution in "virtual" mode, that is, without requesting any locks, prefetching data for its second execution phase. The second execution phase is shorter since no disk I/O is required, resulting in a lower effective degree of txn concurrency and less data contention. This method is effective provided access invariance prevails; that is, txns access the same set of objects in the second phase as they did in the first. The optimistic die method is appropriate for the first phase and the optimistic kill method for further phases. Lock preclaiming instead of the optimistic kill method in the second phase prevents further restarts, which is a weak point of the optimistic CC method due to the quadratic effect, that is, the probability of failed validation increases as the square of txn size. Several two-phase processing methods are described and shown to outperform restart-oriented locking methods in high-contention environments provided adequate hardware resources are available. This tutorial reviews CC methods based on standard locking, restart-oriented locking methods, two-phase processing methods including optimistic CC, and hybrid methods (combining optimistic CC and locking) in centralized systems. Its main goals are as follows: (i) succinctly specify CC methods of interest; (ii) describe models for performance evaluation of CC methods, including new models that alleviate some of the shortcomings of models used in earlier studies; (iii) compare the performance of CC methods; (iv) list insights gained from analytic and simulation studies; (v) review methods to relieve the level of lock contention: special methods for indices and aggregate data; modified txn structures; and relaxed levels of consistency for queries; (vi) survey performance evaluation studies of CC methods; (vii) illustrate the applicability of basic analytic methods to evaluating the performance of CC methods; and (viii) suggest areas of further investigation.



ACM Computing Surveys