Browse Theses

In database management systems, transactions are provided for constructing programs which appear to execute atomically. If more than one transaction is allowed to run at once, a concurrency control algorithm must be employed to properly synchronize their execution. Many concurrency control algorithms have been proposed, and this thesis examines the costs and performance characteristics associated with a number of these algorithms.

Two models of concurrency control algorithms are described. The first is an abstract model which is used to evaluate and compare the relative storage and CPU costs of concurrency control algorithms. Three algorithms, two-phase locking, basic timestamp ordering, and serial validation, are evaluated using this model. It is found that the costs associated with two-phase locking are at least as low as those for the other two algorithms.

The second model is a simulation model which is used to investigate the performance characteristics of concurrency control algorithms. Results are presented for seven different algorithms, including four locking algorithms, two timestamp algorithms, and serial validation. All performed about equally well in situations where conflicts between transactions were rare. When conflicts were more frequent, the algorithms which minimized the number of transaction restarts were generally found to be superior. In situations where several algorithms each restarted the same number of transactions, those which restarted transactions which had done less work tended to perform the best.

Two previously proposed schemes for improving the performance of concurrency control algorithms, multiple versions and granularity hierarchies, are also examined. A new multiple version algorithm based on serial validation is presented, and performance results are given for this algorithm, the CCA version pool algorithm, and multiversion timestamp ordering. Unlike their single version counterparts, all three algorithms performed comparably under the workloads considered. Three new hierarchical concurrency control algorithms, based on serial validation, basic timestamp ordering, and multiversion timestamp ordering, are presented. Performance results are given for these algorithms and a hierarchical locking algorithm. All were found to improve performance in situations where the cost of concurrency control was high, but were of little use otherwise.